Optics Questions

1. A camera has a lens with a focal length of 26.9 cm. When an object is placed 3.61 meters away is being photographed, how far from the film should the lens be placed? give your answer cm.

Answer is below.

2. When an object is viewed through a magnifying glass, an object 3.1 cm high appears as an upright image 13.9 cm high. What is the magnification? There are no units in this problem, since magnification is simply a number.

Answer is below.

3. A lens that is thicker at the center than the edges

a) is a converging lens

4. Two otherwise identical light waves arriving at the same point with peak matching valley undergo

b) constructive interference

5. Parallel light rays from a distant object strike the surface of a concave mirror. After reflecting from the mirror’s surface, the rays will converge at

c) the mirror’s focal point.

6. According to the law of reflection, a light ray striking a mirror at an angle of incidence of 30 degree will leaves the mirror at an an angle of

b) 30 degrees

7. If you see a rainbow near the time of sunset, where in the sky will the rainbow be?

b) towards the eastern horizon

Answers to 1 and 2.


The equation of for the position of the object $(x_1)$, of the image $(x_2)$ and of the focal distance $(f)$ for a lens is

$(1/x_1) +(1/x_2) =1/f$

where $x_1$ is positive to the right of the lens (real object) , $x_2$ is positive to the left of the lens (real image) and $f$ is positive for converging lens.

$1/0.269 = (1/3.61) +(1/x_2)$

$1/x_2 = 3.71 – 0.277$

$x_2 =0.29 m$

2.The magnification of a lens is simply defines as the

$M = h(image)/h(object)$ . Since the image and the object are both real and upright then the magnification is

$M=13.9/3.1 =4.48$