JAMB Physics Past Questions & Answers - Page 299

1,493.

If the focal length of a camera lens is 20cm, the distance from the film at which the lens must be set to produce a sharp image of an object 100cm away is

A.

17cm

B.

20cm

C.

25cm

D.

100cm

Correct answer is C

F = \(\frac{uv}{u + v}\)

20 = \(\frac{100 \times v}{100 + v}\)

v = 25cm

1,494.

Which of the following expressions gives the linear magnification produced by a concave mirror of radius of curvature r, if U and V are the object and image distances respectively?

A.

\(\frac{V}{r}\) - 1

B.

\(\frac{2V}{r}\) - 1

C.

\(\frac{U}{r}\) - 1

D.

\(\frac{2U}{r}\) - 1

Correct answer is B

F = \(\frac{r}{2}\) and v = mu

F = \(\frac{UV}{U + V}\)

\(\frac{r}{2}\) = \(\frac{U \times mu}{U + mu}\)

\(\frac{r}{2}\) = \(\frac{mu^2}{u(1 + m)}\)

\(\frac{r}{2} = \frac{mu}{1+m}\)

\(\frac{r}{2} = \frac{v}{1+m}\)

\(\frac{2v}{r} = 1+m\)

\(m = \frac{2v}{r} - 1\)

1,495.

Radio waves emitted from an antenna are picked up by a radar after reflection from an aircraft in 4 x 10-3S. How far is the aircraft from the antenna? [c = 3 x 108ms-1 ]

A.

6.0 x 102ms-1

B.

1.2 x 103km

C.

3.0 x 103km

D.

6.0 x 10 5km

Correct answer is D

Time = 1 x 4 x 10-3sec

distance = velocity x time

= 3 x 108 x 4 x 10-3

= 6 x 105m