One mark questions with answers
Q1. Name the mirror, which always produces a virtual, erect and diminished image of an object.

Ans1. Convex mirror always produces virtual, erect and diminished image. It is always obtained behind the mirror.

Q2. A candle is placed in front of a concave mirror at a distance of 30cm. Its real image is obtained on a screen 15cm away from the mirror. What is the magnification?

Ans2. Magnification, m = -v/u = +15/-30 = - 0.5. The image is real and inverted.

Q3. A 4cm high object is placed at a distance of 2f (where f is a focal length) from a convex lens. What is the height of the image formed?

Ans3. 4cm because when the object is at the center of curvature the image is obtained at the same place and is of the same size.

Q4. The focal length of a convex lens is 25cm. What is its power?

Ans4. Power, P = 100/f dioptre = + 4 D.


Two mark questions with answers

Q1. Why does it take some time to see objects in dim light when you enter the room from bright sunlight outside?

Ans1. The pupil controls the amount of light entering our eye. It becomes smaller for larger intensity of light and reduces the amount of light entering the eye. When we enter a dimly lit room then the amount of light received by the eye is small and so the pupil expands and takes time before we can see the objects around.

Q2. You have two lenses of focal length 10 cm and 100 cm. By means of mounting them in a metallic tube, give order of lenses to make a telescope
(a) Which lens would you point towards the object?
(b) Where would you put the second lens?

Ans2. (a) Lens of focal length 100 cm will be pointed towards the object.
(b) Lens of focal length 10 cm will be placed on other side of the metallic tube at a distance of 110 cm from it.

Q3. Why does a red flower appear black when seen in green light?

Ans3. This is due to the fact that there is no red colour in green light, which can be reflected by the rose. Since red rose absorbs the green light falling on it so it appears black.

Q4. Why does a red rose appear red in colour?

Ans4. A red rose appears red in colour because out of the seven colours of the light falling on it only red is reflected back while all others are absorbed.

Three mark questions with answers

Q1. How is the amount of light entering the eye controlled? What change is made in the eye to enable it to focus on objects situated at different distances?

Ans1. The amount of light entering the eye is controlled by the pupil. The pupil can change its diameter according to the intensity of light. It becomes smaller for larger intensity of light and larger for smaller intensity of light.
To focus on distant objects, the ciliary muscles are relaxed causing the eye lens to become thin and thus increasing the focal length of the eye lens. To focus on nearby objects, the ciliary muscles contract causing the eye-lens to become thick and thus decreasing the focal length of the eye-lens. In this manner, by changing the focal length of eye-lens, the images of objects at different distances are brought to focus on the retina.

Q2. Define principal focus of spherical mirrors and draw a ray diagram to show the principal focus and focal length of a concave mirror and a convex mirror.

Ans2. Principal focus of a concave mirror : The principal focus of a concave mirror is a point on its principal axis at which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror.
In the figure point F is the principal focus of the concave mirror. The distance PF is the focal length of the concave mirror.

Principal focus of a convex mirror : The principle focus of a convex mirror is a point on its principal axis from which a beam of light rays, initially parallel to the axis, appears to diverge after being reflected from the convex mirror. The point F is the principal focus of the convex mirror and PF is its focal length.

Q3. If an object 5 cm is placed at a distance of 10 cm from a convex mirror of radius of curvature 30 cm, find the nature, position and size of the image.

Ans3. Size of the object, O = 5 cm,
u = -10 cm,
f = R/2 = 30/2 = +15 cm
1/f = 1/u + 1/v
1/v = 1/f - 1/u
= 1/15 - 1/-10 = 1/6
v = +6 cm
Size of image/Size of object = I/O = -v/u
I = (-v/u) x O
= -(6/-10) x 5 = 3 cm
The image is 3 cm high, virtual, erect and at 6 cm from the mirror behind it.

Q4. An object 2 cm high is placed at a distance of 16 cm from a concave mirror, which produces a real image 3 cm high
(i) What is the focal length of the mirror?
(ii) Find the position of the image.

Ans4. Size of the object, O = 2 cm,
u = -16 cm,
Size of the image, I = -3 cm.
-v/u = I/O = m
or -v/-16 = -3/2
or v = 16 x (-3/2) = -24 cm.
The image is formed at a distance of 24 cm in front of the mirror
1/f = 1/v + 1/u
= 1/-24 + 1/-16
= - 1/24 - 1/16 = -5/48
f = -48/5 = -9.6 cm.


Five mark questions with answers

Q1. With the help of a diagram, describe the working of a compound microscope for the formation of the image at finite distance or least distance of distinct vision.

Ans1. Construction: It consists of two convex lenses with an arrangement to adjust distance between them. The lens towards the object is called objective and the lens towards the eye is called eye-piece. Both the lenses are of short focal length. But the focal length of objective is smaller than that of the focal length of eye-piece.

The object to be viewed is kept at a distance slightly larger than the focal length of the objective. The objective forms an inverted, magnified and real image of the object. The eye-piece is fixed in such a way that the image formed by the objective lens lies between the eye-piece and its focus. The image formed by objective is A'B' in the diagram above. This image acts as the object for the eye-piece which forms a further magnified but vertical image of the object. This is shown by A"B". It is this image that we see when we look into a slide through a microscope. The final image seen remains inverted with respect to the object.
The objective and the eye-piece of a microscope are mounted at the ends of a tube. The mounting is done in such a way that their axes are common.
The magnification power of the microscope: The magnification power of the microscope is the ratio of the size of the image and that of the object. The value of magnification is given by

M = D x L/f_e x f_o
Where D is the distance of distinct vision (25cm), L the length of the microscope tube, fe and fo the focal lengths of the eye-piece and objective respectively, in centimetres.

Q2. (a) Draw a ray diagram to show the formation of image by concave lens when the object is in between focal plane and optical centre.
(b) What are the uses of concave mirrors?
(c) What do you mean by f-number of a camera?

Ans. (Try yourself).

Q3. (a) What do you mean by spectrum? Give its types. Show with the help of diagram the splitting and recombination of constituent colours of white light.
(b) What do you understand by the least distance of distinct vision? State its value for a normal eye.

Ans. (Try yourself).