Question

11 Where is the image located when an object is 60 cm from a convex mirror with a focal length of 20 cm?
a.15 cm behind
b. 30 cm behind
c. 60 cm behind
d. 15 cm in front
12 If the sun is 150 million km away from the earth, how long does it take sunlight to reach the earth
a. 0.5 s
b. 1500 s
c. 45 s
d. 500 s
13 The critical angle for total internal reflection at an air-water interface is approximately 48 degrees. In which of the following situations will total internal reflection occur?
a. light incident in water at 40 dgrees
b. light incident in water at 55 dgrees
c. light incident in air at 40 dgrees
d. light incident in air at 55 dgrees
14 How many diopters are there for for a converging lens with a focal length of 0.4m?
a. −2.5
b. −0.4
c. +0.4
d. +2.5

Answer 1

Answer on Question #51685-Physics-Optics

11 Where is the image located when an object is 60 cm from a convex mirror with a focal length of 20 cm?

a. 15 cm behind

b. 30 cm behind

c. 60 cm behind

d. 15 cm in front

Solution

Here $u = -ve, f = +ve$.

$$\frac{1}{u} + \frac{1}{v} = \frac{1}{f}.$$ $$\frac{1}{-60} + \frac{1}{v} = \frac{1}{20} \rightarrow v = 15\,\text{cm}$$

The $+ve$ sign indicates that image is virtual and behind the mirror.

Answer: a. 15 cm behind.

12 If the sun is 150 million km away from the earth, how long does it take sunlight to reach the earth

a. 0.5 s

b. 1500 s

c. 45 s

d. 500 s

Solution

$$d = 150 \cdot 10^6\,k = 150 \cdot 10^9\,m,$$

$v = 3 \cdot 10^8\,\frac{m}{s}$ is the speed of light.

$$\text{time} = \frac{\text{distance}}{\text{speed}}$$

We will divide the distance to the Sun by the speed of light, which will give us the answer in seconds:

$$t = \frac{d}{v} = \frac{150 \cdot 10^9}{3 \cdot 10^8} = 500\,\text{s}.$$

Answer: d. 500 s.

13 The critical angle for total internal reflection at an air-water interface is approximately 48 degrees. In which of the following situations will total internal reflection occur?

a. light incident in water at 40 degrees

b. light incident in water at 55 degrees

c. light incident in air at 40 degrees

d. light incident in air at 55 degrees

Solution

Total internal reflection only takes place when both of the following two conditions are met:

1) a light ray is in the more dense medium and approaching the less dense medium (therefore it is possible only then light incident from water on air)

2) the angle of incidence for the light ray is greater than the critical angle (in this case 48°). Therefore total internal reflection will occur if light incident from water on air at 55°

**Answer:** b. light incident in water at 55 degrees.

14 How many diopters are there for a converging lens with a focal length of 0.4m?

a. -2.5

b. -0.4

c. +0.4

d. +2.5

Solution

A lens with a focal length of $f = +0.4 \, m$ (converging lens) has a power

$$P = \frac{1}{f} = \frac{1}{+0.4 \, m} = +2.5 \, \text{diopters}.$$

**Answer:** d. +2.5

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