Focus and Focal Length

Q.

Define the principal focus of a convex mirror.

Q.

Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

1. A convex lens of focal length 50 cm.

2. A concave lens of focal length 50 cm.

3. A convex lens of focal length 5 cm.

4. A concave lens of focal length 5 cm.

Q.

The principal focus of a convex mirror is virtual.

1. False

2. True

Q.

What do you understand by the focus and focal length of a spherical mirror ? Show them on the separate diagrams for each of a concave mirror and a convex mirror.

Q.

Define focal length

Q.

How is the focal length of a spherical mirror related to its radius of curvature ?

Q.

If the radius of curvature of a concave mirror is 20 cm, then its focal length is:

1. 40 cm

2. 80 cm

3. 10 cm

4. 20 cm

Q. A ray of light travelling in the direction $\frac{1}{2}(i+\sqrt{3j})$ is incident on a plane mirror. After reflection, it travels along the direction $\frac{1}{2}(i-\sqrt{3j})$. The angle of incidence is

1. $30$
2. $60$
3. $45$
4. $75$

Q. A convex lens of ____ focal length gives a greater magnification than lenses of ____ focal length.

1. Long, short
2. Short, equal
3. Equal, short
4. Short, long

Q. What is 2F called in simple words?

Q.

To see the image of an object formed by a lens, the object and the observer should be:

1. on the same side of the lens

2. on the different sides of the lens

3. position of the observer does not matter

4. can’t be said

Q.

If AB = 10 cm and DB = 40 cm (diameter). What is the radius of curvature and focal length?

1. 40 cm, 10 cm

2. 20 cm, 20 cm

3. 20 cm, 10 cm

4. 40 cm, 5 cm

Q. Which of the following relation is true for focal length (f) and the radius of curvature (R)?

1. R = f
2. R = 2 f
3. R = 3 f
4. R = 4 f

Q. A ray of light parallel to principal axis after reflection from concave mirror passes through ___________.

Q.

A spherical concave mirror focuses all rays parallel to its principal axis exactly onto the focus.

1. False

2. True

Q. State the positions of the object in case of image formed by a concave
mirror with the following characteristics : Image is highly magnified

Q.

The focal length of a concave mirror is nearly _________ of its radius of curvature.

Q.

If AO= 10 cm & BO = 40 cm (diameter). What is the radius of curvature & focal length?

1. 40 cm, 10 cm

2. 20 cm, 10 cm

3. 40 cm, 5 cm

4. 20 cm, 20 cm

Q. What do you understand by the focus and focal length of a spherical mirror? Show them on the separate diagrams for each of a concave mirror and a convex mirror.

Q.

Distance between focus and centre of curvature is equal to ___.

1. radius of curvature

2. height of the mirror

3. object distance

4. half of radius of curvature

Q.

What will be the radius of curvature of a spherical mirror whose focal length is 5 cm?

Q. Define the following terms for spherical mirrors-
1. Pole
2. Centre of curvature
3. Radius of curvature
4. Principal Axis
5. Principal focus
6. Focal length

Q.

Where is the image always formed by a concave lens?

1. Between optical centre and focus

2. Between centre of curvature and infinity

3. Between focus and infinity

4. Between centre of curvature and focus

Q. How is the focal length of a spherical mirror related to which radius of curvature

Q. When an object is placed between the focus and the pole of a concave mirror, the image formed is

1. real, inverted and small
2. real, inverted and small size
3. real, inverted and enlarged
4. virtual, erect and enlarged

Q.

What is the relation between curvature and focal length?

Q.

Rays parallel to the principal axis of a concave mirror meet at which point?

1. Centre of curvature

2. Pole

3. Focus

4. At a point in the middle of the centre of curvature and focus

Q.

Which of the following term is not associated with a lens?

1. Aperture

2. Principal focus

3. Efficiency

4. Focal length

Q.

If light source is at focus of concave lens, then after refraction the ray of light will ____________.

1. parallel to the principal axis

2. will appear to be parallel to the principal axis

3. will never be parallel to the principal axis

4. cannot be determined

Q.

The focal length of a concave mirror is 10 cm. Find its radius of curvature.