NCERT Solutions for Class 10th Science Chapter 10 Light Reflection and Refraction
Updated on June 9, 2025 | By Learnzy Academy
Q1. Define the principal focus of a concave mirror.
The principal focus of a concave mirror is a point on the principal axis where light rays that are parallel to the principal axis converge (meet) after reflecting from the concave mirror. Since the rays actually meet at this point, it is called a real focus.
The principal focus lies in front of the concave mirror, and the distance between the mirror's pole (the center of its reflecting surface) and the principal focus is called the focal length.
Q2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
The radius of curvature of a spherical mirror is given as 20 cm.
We know that the focal length of a spherical mirror is half of its radius of curvature.
So,
Focal length = Radius of curvature ÷ 2
Focal length = 20 cm ÷ 2 = 10 cm
Hence the focal length of the mirror is 10 cm.
Q3. Name a mirror that can give an erect and enlarged image of an object.
A concave mirror can give an erect and enlarged image of an object.
This happens when the object is placed between the pole and the principal focus of the concave mirror. It is commonly used in makeup mirrors and shaving mirrors for this reason.
Q4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
We prefer a convex mirror as a rear-view mirror in vehicles because of the following reasons:
- It gives a wider field of view – A convex mirror can cover a larger area behind the vehicle, allowing the driver to see more traffic and surroundings.
- It always forms an erect image – The image formed is upright, which helps the driver understand the position of other vehicles easily.
- The image is diminished (smaller in size) – This allows the mirror to show more objects in a limited space.
These properties make convex mirrors very useful and safe for use as rear-view mirrors in vehicles.
Q5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
The radius of curvature of the convex mirror is given as 32 cm.
We know that the focal length is half of the radius of curvature.
So,
Focal length = Radius of curvature ÷ 2
Focal length = 32 cm ÷ 2 = 16 cm
Since it is a convex mirror, the focal length is taken as positive.
Hence the focal length of the convex mirror is +16 cm.
Q6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Given:
Object distance = 10 cm (in front of the mirror, so take it as –10 cm)
Magnification = 3 times enlarged real image (real image means magnification is negative, so –3)
Using the formula for magnification:
Magnification (m) = Image distance (v) ÷ Object distance (u)
=> –3 = v ÷ (–10)
=> v = 30 cm
Hence the image is located 30 cm in front of the mirror.
Q7. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
When light goes from air into water at an angle, it bends towards the normal (the line straight up from the surface).
This happens because water is thicker (denser) than air, so light slows down and bends closer to the normal.
Q8. Light enters from air into glass which has a refractive index of 1.50. If the speed of light in vacuum is 3 × 10⁸ meters per second, what is the speed of light in the glass?
The speed of light in a medium is equal to the speed of light in vacuum divided by the refractive index of that medium.
Given:
Speed of light in vacuum = 3 × 10⁸ m/s
Refractive index of glass = 1.50
So,
Speed of light in glass = (3 × 10⁸) ÷ 1.50 = 2 × 10⁸ m/s
Therefore, the speed of light in the glass is 2 × 10⁸ m/s.
Q9. You are given kerosene, turpentine and water. In which of these does the light travel fastest?
Light travels fastest in the substance with the lowest refractive index because the refractive index shows how much light slows down in that medium.
Among kerosene, turpentine, and water:
- Water has a refractive index of about 1.33
- Kerosene has a refractive index of about 1.44
- Turpentine has a refractive index of about 1.47
Since water has the lowest refractive index, light travels fastest in water compared to kerosene and turpentine.
Q10. The refractive index of diamond is 2.42. What is the meaning of this statement?
The refractive index of diamond is 2.42 means that light travels 2.42 times slower in diamond than in air. This means when light enters diamond from air, its speed becomes 1 divided by 2.42 times the speed of light in air.
Q11. Define 1 dioptre of power of a lens.
One dioptre is the power of a lens whose focal length is 1 metre.
It is the SI unit of power of a lens.
If the focal length of a lens is 1 metre, then its power is 1 dioptre.
1 dioptre = 1/metre
So, If a lens focuses parallel rays of light at a distance of 1 metre, its power is 1 dioptre.
Q12. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. The image is equal in size to the object, which means the object is placed at a distance of 2F from the lens. So, the object is also placed 50 cm in front of the lens.
Using the lens formula:
1/f = 1/v - 1/u
Here,
v = +50 cm (real image, so positive)
u = -50 cm (object is in front of the lens, so negative)
So 1/f = 1/50 - (-1/50)
=> 1/f = 1/50 + 1/50 = 2/50 = 1/25
So, the focal length f = 25 cm
Now,
Power of the lens (P) = 100 / focal length (in cm)
P = 100 / 25 = 4 dioptres
Hence:
The needle is placed 50 cm in front of the lens.
The power of the lens is +4 dioptres.
Q13. Find the power of a concave lens of focal length 2 m.
Focal length of the concave lens = 2 m
Since it is a concave lens, the focal length is negative.
So, f = –2 m = –200 cm
Using the formula:
Power (P) = 100 / focal length in cm
P = 100 / (–200) = –0.5 dioptres
Hence the power of the concave lens is –0.5 dioptres.
Q14. Which one of the following materials cannot be used to make a lens? (a) Water (b) Glass (c) Plastic (d) Clay
Correct Answer: (d) Clay
To make a lens, the material must be transparent so that light can pass through and bend (refract).
- Water, glass, and plastic are transparent and can be used to make lenses.
- Clay is opaque and does not allow light to pass through, so it cannot be used to make a lens.
Q15. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus.
Correct Answer: (d) Between the pole of the mirror and its principal focus
Explanation:
A concave mirror forms a virtual, erect, and enlarged image only when the object is placed between the pole (P) and the principal focus (F). In this case, the image is formed behind the mirror, is upright, and larger than the object.
Q16. Where should an object be placed in front of a convex lens to get a real image of the size of the object? (a) At the principal focus of the lens (b) At twice the focal length (c) At infinity (d) Between the optical centre of the lens and its principal focus.
Correct Answer: (b) At twice the focal length
Explanation:
When an object is placed at twice the focal length (2F) in front of a convex lens, the lens forms a real, inverted image that is equal in size to the object and is formed at 2F on the other side of the lens.
Q17. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be (a) only plane. (b) only concave. (c) only convex. (d) either plane or convex.
Correct Answer: (d) either plane or convex
Explanation:
- In a plane mirror, the image is always erect and the same size as the object, no matter how far you stand.
- In a convex mirror, the image is always erect, but smaller than the object, and visible no matter how far you stand.
- A concave mirror can produce inverted images if the object is beyond the focus. So, the image is not always erect.
Therefore, the mirror is either plane or convex.
Q18. Which of the following lenses would you prefer to use while reading small letters found in a dictionary? (a) A convex lens of focal length 50 cm. (b) A concave lens of focal length 50 cm. (c) A convex lens of focal length 5 cm. (d) A concave lens of focal length 5 cm.
Correct Answer: (c) A convex lens of focal length 5 cm.
Explanation:
- While reading small letters, we need a lens that can magnify the letters. A convex lens with a small focal length (like 5 cm) has a high power and can magnify small objects well.
- A convex lens converges light and can form a magnified, virtual image when the object is within its focal length.
- A concave lens always produces a smaller, virtual image, so it is not suitable for magnifying small letters.
Hence, a convex lens of focal length 5 cm is preferred.
Q19. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object?
The focal length of the concave mirror is 15 cm.
To get an erect image using a concave mirror, the object must be placed between the pole and the principal focus of the mirror. So, the object should be placed less than 15 cm from the mirror.
The image formed will be virtual and erect.
Also, the image will be larger than the object.