Optics Practice Test

By — McGraw-Hill Professional
Updated on Sep 12, 2011

Review the following concepts if needed:

Optics Practice Test

A good score is eight correct. Answers are given at the end.

1. A simple convex lens has a focal length that varies slightly depending on the wavelength of the light passing through it. When such a lens is used as the objective of a telescope, this effect results in

(a) dispersion.

(b) spherical aberration.

(c) chromatic aberration.

(d) nothing! The premise is wrong. A convex lens has the same focal length for all wavelengths of light passing through it.


2. Suppose that a microscope has an objective whose focal length is 1.00 mm and an eyepiece whose focal length is 25.0 mm. What is the magnification?

(a) 25×

(b) 625×

(c) 0.0400×. This device doesn’t magnify. It makes the specimen look smaller.

(d) We need more information to calculate the magnification.


3. Suppose that a pane of crown glass, with a refractive index of 1.52, is immersed in water, which has a refractive index of 1.33. A ray of light traveling in the water strikes the glass at 45° relative to the normal and travels through the pane. What angle, relative to the normal, will the ray of light subtend when it leaves the pane and reenters the water?

(a) 38°

(b) 54°

(c) 45°

(d) No angle at all! The premise is wrong. The light will never enter the glass. It will be reflected when it strikes the glass surface.


4. Suppose that the numerical aperture of a microscope objective in air is 0.85. The medium between the lens and the specimen is replaced by water, which has a refractive index of 1.33. The numerical aperture of the objective

(a) does not change.

(b) increases to 1.13.

(c) decreases to 0.639.

(d) cannot be calculated from this information.


5. According to the law of reflection,

(a) a ray of light traveling from a medium having a low refractive index to a medium having a higher refractive index is reflected at the boundary.

(b) a ray of light traveling from a medium having a high refractive index to a medium having a lower refractive index is reflected at the boundary.

(c) a ray of light always reflects from a shiny surface in a direction exactly opposite the direction from which it arrives.

(d) none of the above.


6. A Cassegrain-type reflecting telescope has an objective mirror with a diameter of 300 mm and an eyepiece with a focal length of 30 mm. The magnification is

(a) 100×.

(b) 10×.

(c) 9,000×.

(d) impossible to calculate from this information.


7. A diverging lens

(a) can collimate converging rays of light.

(b) can focus the Sun’s rays to a brilliant point.

(c) is also known as a convex lens.

(d) is ideal for use as the objective in a refracting telescope.


8. Suppose that the speed of red visible light in a certain transparent medium is 270,000 km/s. What, approximately, is the index of refraction for this substance with respect to red light?

(a) 0.900

(b) 1.11

(c) 0.810

(d) It cannot be calculated from this information.


9. As the magnification of a telescope is increased,

(a) the image resolution decreases in direct proportion.

(b) physical stability becomes more and more important.

(c) the light-gathering area increases in direct proportion.

(d) dimmer and dimmer objects can be seen.


10. What is the critical angle of light rays inside a gem whose refractive index is 2.4? Assume that the gem is surrounded by air.

(a) 25°

(b) 65°

(c) 67°

(d) 90°



1. c

2. d

3. c

4. b

5. d

6. d

7. a

8. b

9. b

10. a

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