Ch.11: Reflection of Light

Reflection of Light | Class 9 | Science | Chapter 11 | Maharashtra State Board

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Questions & Answers

1. Answer the following questions.

a. Explain the difference between a plane mirror, a concave mirror and a convex mirror with respect to the type and size of the images produced.

Ans.

	Plane mirror	Concave mirror	Convex mirror
Type of image	Virtual and Erect	Virtual (erect) as well as Real (inverted)	Virtual and Erect
Size of image	Same size	Diminished, Same size and magnified	Diminished

b. Describe the positions of the source of light with respect to a concave mirror in

1. Torch light

2. Projector lamp

3. Floodlight

Ans.

(a) Torch light: The source of light is placed at the focus.

(b) Projector lamp : The source of light is placed at the centre of curvature.

(c) Flood light : The source of light is placed just beyond the centre of curvature.

c. Why are concave mirrors used in solar devices?

Ans.

• Solar devices like solar cooker or solar water heater use solar energy to cook food or heat water.

• When sun rays fall on the concave mirror, they converge and come together in the focal plane.

• Due to convergence, the intensity of sun rays increases and the food or water is heated faster. Hence, concave mirrors are used in solar- devices.

d. Why are the mirrors fitted on the outside of cars convex?

Ans.

• A convex mirror is used as rear view mirror because they form erect, virtual, and diminished images.

• This, allows the driver to view a large area in a small mirror.

e. Why does obtaining the image of the sun on a paper with the help of a concave mirror burn the paper?

Ans.

• When sunrays fall on the concave mirror, they converge and come together in the focal plane.

• Due to convergence, the intensity of sunrays increases.

• Hence, image of the sun on a paper with the help of concave mirror bums the paper.

f. If a spherical mirror breaks, what type of mirrors are the individual pieces?

Ans.

• When a spherical mirror breaks into smaller pieces, the radius of curvature and focal length does not change.

• Hence, it will continue to behave like a spherical mirror only.

2. What sign conventions are used for reflection from a spherical mirror?

Ans.

According to the Cartesian sign convention, the pole of the mirror is taken as the origin. The principal axis is taken as the X-axis of the frame of reference. The sign conventions are as follows.

1. The object is always kept on the left of the mirror. All distances parallel to the principal axis are measured from the pole of the mirror.

2. All distances measured towards the right of the pole are taken to be positive, while those measured towards the left are taken to be negative.

3. The distance measured vertically upwards from the principal axis are taken to be positive.

4. The distance measured vertically downwards from the principal axis are taken to be negative.

5. The focal length of a concave mirror is negative while that of a convex mirror is positive.

3. Draw ray diagrams for the cases of images obtained in concave mirrors as described in the table on page 122.

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Intext Questions and Answers

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