Light – Reflection and Refraction — Class 10 Science
Science · Chapter 9
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Chapter summary
Key ideas & terms
- Laws of reflection
- Angle of incidence equals angle of reflection; rays and normal are coplanar.
- Refractive index
- A measure of how much a medium bends light.
- Mirror/lens formula
- Relation linking object distance, image distance and focal length.
- Power of a lens
- The reciprocal of focal length (in metres), measured in dioptres.
Extra questions & answers
Why does a ray of light bend when it enters water from air?
Light slows down in the denser medium, so it bends towards the normal — this is refraction.
What kind of mirror is used as a rear-view mirror in vehicles, and why?
A convex mirror, because it gives an erect, diminished image and a wider field of view.
State the laws of reflection of light.
The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Where should an object be placed before a concave mirror to get a magnified, virtual image?
Between the pole and the focus; the image formed is virtual, erect and larger than the object (refer to the ray diagram in your NCERT textbook).
Why is a convex mirror used as a rear-view mirror in vehicles?
Because it always forms an erect, diminished image and gives a wider field of view of the traffic behind.
Write the mirror formula and the magnification formula.
The mirror formula is
What is refraction of light, and why does it occur?
The bending of light as it passes from one transparent medium to another; it occurs because the speed of light changes in different media.
What is the refractive index of a medium?
A measure of how much a medium slows down and bends light, given by the ratio of the speed of light in vacuum to its speed in the medium.
Write the lens formula and explain the power of a lens.
The lens formula is
How does a convex lens form a real, inverted image, and where is it used?
When the object is beyond the focus, a convex lens forms a real, inverted image on the other side; such lenses are used in cameras, projectors and the human eye (refer to the ray diagram in your NCERT textbook).
QUESTIONS
1. Define the principal focus of a concave mirror.
The principal focus of a concave mirror is the point on its principal axis at which rays parallel to the principal axis meet after reflection from the mirror.
2. The radius of curvature of a spherical mirror is
For a spherical mirror,
3. Name a mirror that can give an erect and enlarged image of an object.
A concave mirror can give an erect and enlarged image when the object is placed between its pole and principal focus.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
A convex mirror is preferred as a rear-view mirror because it always forms an erect and diminished image and provides a wider field of view. It therefore enables the driver to see a larger area of the traffic behind the vehicle.
QUESTIONS
1. Find the focal length of a convex mirror whose radius of curvature is
For a spherical mirror,
2. A concave mirror produces three times magnified (enlarged) real image of an object placed at
For a real, inverted image, the magnification is negative. Therefore,
QUESTIONS
1. 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?
The light ray bends towards the normal because it travels from air, which is optically rarer, into water, which is optically denser. Its speed decreases on entering water.
2. Light enters from air to glass having refractive index
The refractive index is given by
3. Find out, from Table 9.3, the medium having highest optical density. Also find the medium with lowest optical density.
Diamond has the highest optical density because it has the highest refractive index,
4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 9.3.
Light travels fastest in the medium with the lowest refractive index. The refractive indices of water, kerosene and turpentine oil are
5. The refractive index of diamond is
It means that the ratio of the speed of light in air or vacuum to its speed in diamond is
QUESTIONS
1. Define
One dioptre is the power of a lens whose focal length is
2. A convex lens forms a real and inverted image of a needle at a distance of
A convex lens forms a real image equal in size to the object when the object is placed at twice the focal length. Since the image is formed at
3. Find the power of a concave lens of focal length
The focal length of a concave lens is negative, so
EXERCISES
1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay
(d) Clay. A lens must be made of a transparent material so that light can pass through and be refracted. Clay is opaque.
2. 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.
(d) Between the pole of the mirror and its principal focus. In this position, a concave mirror forms a virtual, erect and enlarged image behind the mirror.
3. 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.
(b) At twice the focal length. An object placed at
4. A spherical mirror and a thin spherical lens have each a focal length of
(a) both concave.
(b) both convex.
(c) the mirror is concave and the lens is convex.
(d) the mirror is convex, but the lens is concave.
(a) Both concave. Under the New Cartesian Sign Convention, a concave mirror and a concave lens both have negative focal lengths.
5. 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.
(d) Either plane or convex. A plane mirror always forms an erect image, and a convex mirror also always forms an erect image for every object position.
6. 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
(b) A concave lens of focal length
(c) A convex lens of focal length
(d) A concave lens of focal length
(c) A convex lens of focal length
7. We wish to obtain an erect image of an object, using a concave mirror of focal length
The object must be placed between the pole and the principal focus of the concave mirror. Therefore, its distance from the mirror must be less than
8. Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason.
(a) A concave mirror is used in car headlights. When the bulb is placed at its principal focus, the reflected light forms a strong parallel beam.
(b) A convex mirror is used as a side or rear-view mirror because it forms an erect, diminished image and provides a wide field of view.
(c) A concave mirror is used in a solar furnace because it converges sunlight at its principal focus, producing a high temperature.
9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Yes, the lens will still produce a complete image, but the image will be less bright. Every small portion of a convex lens receives rays from all parts of the object and can form the whole image. To verify this, form a sharp image of a bright object on a screen using a convex lens. Cover half of the lens with black paper without changing the positions of the object, lens or screen. The complete image remains visible, but its brightness decreases because fewer light rays pass through the lens.
10. An object
Here,
11. A concave lens of focal length
For a concave lens,
12. An object is placed at a distance of
Here,
13. The magnification produced by a plane mirror is
A magnification of
14. An object
For the convex mirror,
15. An object of size
Here,
16. Find the focal length of a lens of power
Using
17. A doctor has prescribed a corrective lens of power
Using