WAEC Physics Past Questions & Answers - Page 179

892.

A converging lens produces an image four times as large as an object placed 25cm from the lens. Calculate its focal length

A.

100cm

B.

33cm

C.

29cm

D.

20cm

Correct answer is D

M = \(\frac{v}{u}\)

v = mu = 4 x 25 = 100m

\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f}\)

\(\frac{1}{100} + \frac{1}{25} = \frac{1}{f}\)

f = 20cm

893.

An object is placed on the principal axis and at the centre of curvature of a concave mirror, the image of the image formed by the mirror is

A.

real and magnified

B.

real and inverted

C.

erect and magnified

D.

erect and virtual

Correct answer is B

No explanation has been provided for this answer.

894.

The image which cannot be formed on a screen is said to be

A.

inverted

B.

erect

C.

real

D.

virtual

Correct answer is D

Any image that can be formed on a screen are called real image. Such images are formed by light rays emanating from the object, when reflected and/or refracted by devices such as lenses and mirrors converge at some specific point.

In contrast there are other images which are formed by the rays of light reaching the observer appearing to converge at some point. This happens when the rays reaching the observer are diverging and when the line of these rays are projected backward. Such images, which cannot be formed on a screen, are called virtual image.

895.

Which of the following condition is necessary for the occurrence of total internal reflection of light?

A.

light must travel from an optical less dense to a denser medium

B.

the angle of incidence must be equal to the critical angle

C.

the angle of incidence must be greater than the critical angle

D.

the angle of refraction must be 90o

Correct answer is C

No explanation has been provided for this answer.