Mirror & Lens Formula

A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.

• u=-60; h' =(5/4)

• u=-60; h' =(4/5)

• u= +60; h' =(5/4)

• u=+60; h' =(4/5)

A concave mirror of focal length 20 cm forms an image having twice the size of object. For the virtual position of object, the position of object will be at:

• 25 cm

• 40 cm

• 10 cm

• At infinity

If a man’s face is 25 cm in front of concave shaving mirror producing erect image 1.5 times the size of face, focal length of the mirror would be:

• 75 cm

• 25 cm

• 15 cm

• 60 cm

The refractive index of water is 1.33. The speed of light in water will be:

• 1.33 × 10^8 m/s

• 3 × 10^8 m/s

• 2.66 × 10^8 m/s

• 2.26 × 10^8 m/s

You are given three media A, B and C of refractive index 1.33, 1.65 and 1.46. The medium in which the light will travel fastest is

• A

• B

• C

• Equal in all media

A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is

• -10 cm

• 20 cm

• -30 cm

• -40 cm

Which of the following statements is/are true?

• A convex lens has 4 dioptre power having a focal length 0.25 m

• A convex lens has -4 dioptre power having a focal length 0.25 m

• A concave lens has 4 dioptre power having a focal length 0.25 m

• A concave lens has – 4 dioptre having a focal 0.25 m

A point object is placed at a distance of 20 cm from a convex mirror of focal length 20 cm. The image will form at:

• at infinity

• at focus

• at pole

• behind the mirror.

If the power of a lens is – 2 D, what is its focal length?

• +50 cm

• -100 cm

• -50 cm

• +100 cm

A ray of light is travelling from a rarer medium to a denser medium. While entering the denser medium at the point of incidence, it:

• goes straight into the second medium

• bends towards the normal

• bends away from the normal

• does not enter at all