WBBSE Class 10 Physical Science Notes Chapter 5 Light | WBBSE Class 10 Physical Science Chapter 5 Notes in ENGLISH
WBBSE Class 10 Physical Science Chapter 5 Question Answer – Light
### Very Short Answer Type Questions:
**Question 1.**
*On normal incidence of a ray of light on a plane mirror, what are the angle of incidence and angle of reflection?*
**Answer:**
The angle of incidence (∠i) and angle of reflection (∠r) are both 0°.
**Question 2.**
*What is the linear magnification of an image in a plane mirror?*
**Answer:**
The linear magnification (m) is equal to 1, as the size of the image (I) is equal to the size of the object (O).
**Question 3.**
*Name the type of mirror which has a focal length equal to infinity.*
**Answer:**
A plane mirror.
**Question 4.**
*What is lateral inversion of an image?*
**Answer:**
Lateral inversion occurs when the side of an object appears as left in the image and vice versa in an image formed in a plane mirror.
**Question 5.**
*What is the radius of curvature and focal length of a plane mirror?*
**Answer:**
In a plane mirror, both the radius of curvature and focal length are infinity.
**Question 6.**
*A man approaches a vertical plane mirror at a speed u. At what speed does he approach his image?*
**Answer:**
The man approaches his image with a speed = 2u.
**Question 7.**
*Define the principal axis of a spherical mirror.*
**Answer:**
The principal axis is the straight line passing through the pole and the center of curvature of the mirror.
**Question 8.**
*Why is a convex mirror used as a driver’s mirror?*
**Answer:**
A convex mirror is used as a driver's mirror because it always produces an erect image of diminished size, providing a wide field of view.
**Question 9.**
*Which mirror is convergent, concave, or convex?*
**Answer:**
A concave mirror is convergent.
**Question 10.**
*Can we cast the image formed by a convex mirror on a screen?*
**Answer:**
No, the image of a real object cannot be cast, but the image of a virtual object can be cast.
**Question 11.**
*Can a convex mirror form a magnified image?*
**Answer:**
No.
**Question 12.**
*When does a concave mirror form a virtual image?*
**Answer:**
A concave mirror forms a virtual image when the object lies between the pole and focus of the mirror.
**Question 13.**
*What is the relation between f and r of a spherical mirror?*
**Answer:**
The relation is f = r/2.
**Question 14.**
*Can a virtual image be photographed?*
**Answer:**
Yes, a virtual image can be photographed with the help of the lens of the camera.
**Question 15.**
*What is the number of images of an object held between two parallel plane mirrors?*
**Answer:**
The number of images (n) is given by n = (360/θ) − 1 = α when θ = 0°.
**Question 16.**
*What is the effect of the size of a mirror on the nature of the image?*
**Answer:**
The nature of the image does not depend on the size of the mirror.
**Question 17.**
*Does the focal length of a concave mirror change if it is immersed in water?*
**Answer:**
No.
**Question 18.**
*Which of the following does not change when light goes from one medium to another: frequency, wavelength, speed, and intensity?*
**Answer:**
Frequency.
**Question 19.**
*What is the absolute refractive index of vacuum?*
**Answer:**
1.
**Question 20.**
*For which medium is refractive index (i) minimum, and (ii) maximum?*
**Answer:**
(i) μ_min for vacuum (=1) and (ii) μ_max for diamond (=2.42).
**Question 21.**
*Can total internal reflection occur when light travels from a rarer to a denser medium?*
**Answer:**
No.
**Question 22.**
*Does the critical angle depend on the color of light?*
**Answer:**
Yes, as the refractive index is different for different colors of light.
**Question 23.**
*What is the cause of refraction of light?*
**Answer:**
Different velocities of light in different media cause refraction.
**Question 24.**
*Give the relation between the critical angle and refractive index.*
**Answer:**
The relation is θ_c = sin^(-1)(1/μ).
**Question 25.**
*Can light traveling from air to glass suffer total internal reflection?*
**Answer:**
No.
**Question 26.**
*What is the principle of optical fiber?*
**Answer:**
The principle is total internal reflection of light.
**Question 27.**
*What do you mean by refraction of light?*
**Answer:**
Refraction of light is the phenomenon of bending light rays from their original path while passing from one medium to another.
Very Short Answer Type Questions:
WBBSE Class 10 Physical Science Chapter 5 Notes in ENGLISH
**Question 50.**
*What is the cause of dispersion?*
**Answer:**
Dispersion is caused by the difference in the deviations suffered by the two extreme colors (red and violet) of white light passing through a prism, resulting in angular dispersion.
**Question 51.**
*What is the cause of dispersion?*
**Answer:**
The deviations of different colors of light through a prism are different, leading to dispersion.
**Question 52.**
*What is the unit of angular dispersion?*
**Answer:**
The unit of angular dispersion is degrees.
**Question 53.**
*What is the dispersive power of a prism?*
**Answer:**
The dispersive power of a prism is the ratio of angular dispersion to the mean deviation produced by the prism.
**Question 54.**
*Does dispersive power of a prism depend on the angle of the prism?*
**Answer:**
No, it depends on the nature of the material of the prism.
**Question 55.**
*What is the essential condition for observing a rainbow?*
**Answer:**
The sun should be at the back of the observer after rainfall.
**Question 56.**
*Can a short-sighted person read books without spectacles?*
**Answer:**
Yes, a short-sighted person can see objects at a short distance.
**Question 57.**
*What kind of lens one should use to correct a myopic eye?*
**Answer:**
A concave lens.
**Question 58.**
*What kind of lens one should use to correct an astigmatic eye?*
**Answer:**
A cylindrical lens.
**Question 59.**
*What is the least distance of distinct vision?*
**Answer:**
25 cm.
**Question 60.**
*What is the far point of a normal eye?*
**Answer:**
Infinity.
**Question 61.**
*Which color of light travels faster in a vacuum?*
**Answer:**
Light rays of all colors travel with the same speed in a vacuum.
**Question 62.**
*How many colors of light mix to produce white light?*
**Answer:**
Seven colors of light mix to produce white light.
**Question 63.**
*Will the focal length of a convex lens vary for refraction of light of different colors through it?*
**Answer:**
Yes, for different colors of light, the convex lens will have different focal lengths.
**Question 64.**
*What will be the position and nature of the image formed with a convex lens when the object is situated at a distance twice the focal length of the lens?*
**Answer:**
The image will be at a distance twice the focal length on the other side of the lens, and the nature of the image will be real, inverted, and the same size as that of the object.
**Question 65.**
*What is a convex lens?*
**Answer:**
If the lens is thicker at its center than at its edges, it is a convex or converging lens.
**Question 66.**
*Where would you place an object before a convex lens to get a virtual and magnified image?*
**Answer:**
If the object is at a distance less than the focal length of the lens, the image becomes virtual, erect, and magnified.
**Question 67.**
*Who discovered first the dispersion of light?*
**Answer:**
Dispersion of light was first discovered by Sir Isaac Newton in 1666.
**Question 68.**
*What is dispersion of light?*
**Answer:**
The splitting up of white light into seven colors on passing through a refracting medium like a prism is known as the dispersion of light.
**Question 69.**
*What is a spectrum?*
**Answer:**
The band of different colors obtained due to the dispersion of white light is known as a spectrum.
**Question 70.**
*What is a lens?*
**Answer:**
A lens is a piece of transparent refracting substance bounded by two spherical or one spherical and another plane surfaces.
**Question 71.**
*What is a concave lens?*
**Answer:**
The lens which is thinner at the center and wider at the two edges is called a concave lens.
**Question 72.**
*What is a prism?*
**Answer:**
A homogeneous transparent refracting medium enclosed by three rectangular and two triangular surfaces is known as a prism.
**Question 73.**
*What is the principal section of the prism?*
**Answer:**
The surface that intersects the refracting surface of the prism perpendicularly is known as the principal section of the prism.
**Question 74.**
*What is the refracting angle of the prism?*
**Answer:**
The angle made by the two refracting surfaces of the prism is known as the refracting angle of the prism.
**Question 75.**
*What is polychromatic light?*
**Answer:**
The light which is composed of more than one color is known as polychromatic light.
**Question 76.**
*What is monochromatic light?*
**Answer:**
The light which consists of only one color is known as monochromatic light.
**Question 77.**
*What are primary colors?*
**Answer:**
Red, green, and blue are called primary colors.
**Question 78.**
*Give the name of a secondary color.*
**Answer:**
Yellow is called a secondary color.
**Question 79.**
*What is linear magnification?*
**Answer:**
The ratio of the length of the image to that of the object is called the linear magnification.
**Question 80.**
*What is the focal plane of the lens?*
**Answer:**
The plane passing through the principal focus of a lens and perpendicular to the principal axis of the lens is known as the focal plane of the lens.
**Question 81.**
*How does a green cloth appear in red light?*
**Answer:**
The green cloth appears black in red light.
**Question 82.**
*What is a double convex lens?*
**Answer:**
If both surfaces of a convex lens are convex, then the lens is called a double convex lens. It is used in cameras, telescopes, etc.
**Question 83.**
*What is a plano-convex lens?*
**Answer:**
If one surface of a convex lens is plane and the other is convex, then it is called a plano-convex lens. e.g., eye piece of a telescope.
**Question 84.**
*What is a concavo-convex lens?*
**Answer:**
If one surface of a convex lens is convex and the other is concave, then it is called a concavo-convex lens. It is used in spectacles.
**Question 85.**
*What color of light is used for a danger signal?*
**Answer:**
Red.
**Question 86.**
*Name a phenomenon occurring in nature due to the dispersion of light.*
**Answer:**
Rainbow.
**Question 87.**
*What type of reflection is used in a torch?*
**Answer:**
A torch uses concave reflector with the bulb placed very near to the focus of the reflector.
**Question 88.**
*When a point object is placed in front of a conc
ave mirror in between the center of curvature and focus, where will the image be formed?*
**Answer:**
The image will be formed at a distance greater than the radius of curvature.
**Question 89.**
*What type of mirror is represented by the front side of a shining stainless spoon?*
**Answer:**
The front side of a shining stainless steel spoon acts like a concave mirror.
**Question 90.**
*What type of mirror is represented by the backside of a shining stainless steel spoon?*
**Answer:**
The backside of a shining stainless steel spoon acts like a convex mirror.
Short answer type questions :
**Question 1.**
*What is the centre of curvature of a lens?*
**Answer:**
The centre of curvature of a lens is the center of the sphere from which the surfaces of the lens are cut.
**Question 2.**
*What do you mean by the principal axis of a convex lens?*
**Answer:**
The principal axis of a convex lens is the line joining the centers of curvature of the two spherical surfaces of the lens.
**Question 3.**
*What is the radius of curvature?*
**Answer:**
The radius of curvature of a lens is the radius of the glass sphere from which the surfaces of the lens are cut.
**Question 4.**
*What is the focus of a convex lens?*
**Answer:**
The focus of a convex lens is the point where a beam of parallel rays, traveling parallel to the principal axis, converges after passing through the lens.
**Question 5.**
*What is the optical centre?*
**Answer:**
The optical centre of a lens is the point through which a ray of light passes undeviated when it strikes the lens surface in such a way that the emergent ray is parallel to the incident ray.
**Question 6.**
*What is the focal length?*
**Answer:**
The focal length is the distance between the optical centre and the focus of a lens.
**Question 7.**
*What is the focal plane?*
**Answer:**
The focal plane of a lens is the plane passing through the principal focus of the lens and perpendicular to the principal axis.
**Question 8.**
*What is the optical plane?*
**Answer:**
The optical plane is an imaginary vertical plane that cuts the principal axis perpendicularly and passes through the optical centre of a lens.
**Question 9.**
*What is linear magnification of a lens?*
**Answer:**
The linear magnification of a lens is the ratio of the length of the image to that of the object.
**Question 10.**
*What does the term ‘thin’ signify when related to a convex lens? Do all rays of light suffer deviation while crossing through a thin convex lens?*
**Answer:**
A thin convex lens is one with a thickness at the middle that is extremely small compared to the radii of curvature of its surfaces. Rays directed towards the optical centre do not suffer deviation while crossing through a thin convex lens.
**Question 11.**
*Which rays should be considered in drawing ray diagrams for images formed by a convex lens?*
**Answer:**
Consider the following:
- A beam of rays parallel to the principal axis converges through the principal focus after refraction.
- A beam of rays passing through the principal focus emerges parallel to the principal axis after refraction.
- A beam of rays through the optical centre passes through undeviated.
**Question 12.**
*Draw a neat diagram of the formation of an image of an object at infinity by a convex lens.*
**Answer:**
[Image for Question 12](link to the image)
Explanation:
- Distance of the object: At infinity
- Image formed: On the focal plane
- Nature of image: Real and inverted
- Size of image: Very much smaller than the object.
**Question 13.**
*Draw a neat diagram of the formation of an image of an object placed between infinity and 2f from the lens.*
**Answer:**
[Image for Question 13](link to the image)
Explanation:
- Distance of the object: Between infinity and 2f from the lens.
- Image formed: On the opposite side of the object and is situated between f and 2f.
- Nature of image: Real and inverted.
- Size of image: Smaller than the object in size.
**Question 14.**
*Draw a neat diagram of the formation of an image of an object at 2f from the lens.*
**Answer:**
[Image for Question 14](link to the image)
Explanation:
- Distance of the object: At 2f from the lens.
- Image formed: At a distance 2f from the lens.
- Nature of image: Real and inverted.
- Size of image: Equal to the size of the object.
**Question 15.**
*Draw a neat diagram of the formation of an image of an object placed between ‘f’ and 2f from the lens.*
**Answer:**
[Image for Question 15](link to the image)
Explanation:
- Distance of the object: Between 'f' and 2f from the lens.
- Image formed: On the opposite side of the object at a distance greater than 2f.
- Nature of image: Real and inverted.
- Size of image: Greater in size than the object.
**Question 16.**
*Draw a neat diagram of the formation of an image of an object at the focus of the lens.*
**Answer:**
[Image for Question 16](link to the image)
Explanation:
- Distance of the object: At the focus.
- Image formed: At infinity.
- Nature of image: Real and inverted.
- Size of image: Highly magnified.
**Question 17.**
*Draw a neat diagram of the formation of an image of an object situated between the lens and the focus.*
**Answer:**
[Image for Question 17](link to the image)
Explanation:
- Distance of the object: Within the focal length.
- Image formed: On the same side of the object.
- Nature of image: Virtual and erect.
- Size of image: Magnified.
**Question 18.**
*(i) Draw a neat diagram to explain the principle of a magnifying glass
(ii) To read the tiny scripts of a book with ease, where should the book be kept in front of a magnifying glass?*
**Answer:**
[Image for Question 18](link to the image)
(i) *Explanation:* An erect, virtual, and magnified image is formed on the same side of the object. This property of a convex lens is the working principle of a magnifying glass.
(ii) *To read tiny scripts of a book:* The book must be placed within the focal length of the lens.
**Question 19.**
*What is a spectrum?*
**Answer:**
A spectrum is the band of different colors obtained due to the dispersion of white light.
**Question 20.**
*What is a pure spectrum?*
**Answer:**
A pure spectrum is one in which the constituent colors do not overlap on each other and are separated distinctly into elementary colors.
**Question 21.**
*What is an impure spectrum?*
**Answer:**
An impure spectrum is one in which the constituent colors partially superpose on each other and are not separated distinctly into elementary colors.
**Question 22.**
*What are the differences between a Pure and Impure spectrum?*
**Answer:**
**Distinction between Pure and Impure spectrum:**
| Pure spectrum | Impure spectrum |
| --- | --- |
| (i) In a pure spectrum, all the seven colors occupy distinct and different positions on a screen. | (i) In an impure spectrum, all the seven colors do not occupy different and distinct positions on a screen. |
| (ii) In a pure spectrum, monochromatic colors present in white light are placed in order of refrangibility. | (ii)
In an impure spectrum, monochromatic colors are not placed in order of refrangibility.
**Question 23.**
*Is dispersion possible without refraction?*
**Answer:**
Dispersion without refraction is not possible. In an optical medium, different colors travel with different speeds and thus deviate with different magnitudes. Therefore, the constituent colors of polychromatic light disperse after refraction.
**Question 24.**
*Why is blue used after washing white shirts?*
**Answer:**
Blue whitens yellow or orange colors. After washing, white shirts may have some yellow or orange stains. Using blue helps convert these stains to white.
**Question 25.**
*How would you determine whether a mirror is plane, concave, or convex?*
**Answer:**
- A plane mirror produces an erect image of the same size as that of the object.
- A concave mirror produces an erect and magnified image of an object when placed within its focus.
- A convex mirror produces always an erect and diminished image of an object.
### Numerical Problems:
**Example 1:**
*How far from a concave mirror of radius 2 m would you place the object to get an image magnified 3 times?*
**Answer:**
Given magnification, \( m = \pm 3 \) (real image), \( r = -2 \) m.
Using the mirror formula: \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), where \( f \) is the focal length, \( v \) is the image distance, and \( u \) is the object distance.
For a concave mirror, \( f \) is negative.
Given \( m = \frac{v}{u} = 3 \), and \( r = -2 \) m, for a real image \( v \) will be negative.
Now, \( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \) becomes \( \frac{1}{f} = \frac{1}{(-3u)} - \frac{1}{u} \).
Solving for \( u \), \( u = -2.25 \) m.
Therefore, the object should be placed 2.25 m in front of the concave mirror.
**Example 2:**
*A concave mirror forms an image of a 20 cm high object on a screen placed 5 m away from the mirror. The height of the image is 50 cm. Find the focal length of the mirror and the distance between the mirror and the object.*
**Answer:**
Given size of object \( h_1 = 20 \) cm, size of image \( h_2 = 50 \) cm, \( v = -5 \) m (since the image is formed on the same side as the object), and \( u \) is the distance between the mirror and the object.
Using magnification formula: \( m = \frac{h_2}{h_1} = \frac{v}{u} \), \( m = \frac{50}{20} = 2 \).
Now, using the mirror formula \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), where \( f \) is the focal length, \( v \) is the image distance, and \( u \) is the object distance.
Solving for \( u \), \( u = -10 \) m.
Therefore, the distance between the mirror and the object is 10 m.
**Example 3:**
*Light falls from glass of refractive index 1.5 to air. Find the angle of incidence for which the angle of deviation is 90°.*
**Answer:**
Given refractive index \( \mu = 1.5 \) (glass to air) and angle of deviation \( \delta = 90^\circ \).
The critical angle \( \theta_c \) is given by \( \theta_c = \sin^{-1}\left(\frac{1}{\mu}\right) \).
Substituting \( \mu = 1.5 \), \( \theta_c = \sin^{-1}\left(\frac{1}{1.5}\right) \).
Solving for \( \theta_c \), \( \theta_c \approx 41.8^\circ \).
Since \( \theta_c \) is the angle for total internal reflection, the angle of incidence for \( \delta = 90^\circ \) is half of \( \theta_c \), i.e., \( \theta_i = 45^\circ \).
**Example 4:**
*Where should an object be placed from a converging lens of focal length 20 cm, so as to obtain a real image of magnification 2?*
**Answer:**
Given focal length \( f = 20 \) cm, magnification \( m = 2 \).
Using the lens formula \( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \), where \( f \) is the focal length, \( v \) is the image distance, and \( u \) is the object distance.
Since the lens is converging, \( f \) is positive.
Given \( m = \frac{v}{u} = 2 \), for real image \( v \) will be positive.
Now, \( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \) becomes \( \frac{1}{20} = \frac{1}{v} - \frac{1}{u} \).
Solving for \( u \), \( u = 10 \) cm.
Therefore, the object should be placed 10 cm in front of the converging lens.
**Example 5:**
*Two thin lenses of focal lengths 15 cm and 30 cm, respectively, are kept in contact with each other. What is the power of the combined system?*
**Answer:**
Given focal lengths \( f_1 = 15 \) cm and \( f_2 = 30 \) cm.
The formula for the focal length \( F \) of the equivalent lens for two lenses in contact is \( \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} \).
Substituting \( f_1 = 15 \) cm and \( f_2 = 30 \) cm, \( \frac{1}{F} = \frac{1}{15} + \frac{1}{30} \).
Solving for \( F \), \( F = 10 \) cm.
The power \( P \) of the lens is given by \( P = \frac{1}{F} \).
Substituting \( F = 10 \) cm, \( P = \frac{1}{10} \).
Therefore, the power of the combined system is \( 0.1 \) D.
