Question

An object is attached to the bottom of a glass plate with index of refraction 1.5. Three light rays (a, b, c) emanating from the object are shown; they make angles of 0o (a), 30o (b), and 60o (c) with the normal. An air-filled gap separates the glass plate from the front lens of an optical instrument that is used to view the object. The lens is also made of glass with index of refraction 1.5 We are interested in the paths of the rays from the object into the front lens (how they leave is not our concern).

a)

When is total internal reflection at a glass/air interface possible?

Question 17 options:

	For light striking the interface from the glass side.
	For light striking the interface from the air side.
	Never

b)

What happens to ray a as it passes from the glass plate through the gap into the front lens?

Question 18 options:

	Ray a is refracted to the right at both interfaces.
	Ray a undergoes total internal reflection.
	Ray a is refracted to the left at the first interface and to the right at the second.
	Ray a goes straight up through both interfaces.

c)

Write an equation for the angle that ray b makes with the normal to the glass-air interface inside the gap. Then calculate the value in degrees.

d)

For ray b, what is the angle with the normal inside the front lens?

Question 20 options:

	smaller than 30 degrees
	larger than 30 degrees but smaller than 60 degrees
	60 degrees
	30 degrees

e)

What is going to happen to ray c? Give a brief description of its path.

f)

Now assume that the gap is filled with oil with a matching index of refraction, 1.5.

(a) What will happen to the rays now?

(b) What does this imply for the brightness of the object's image when viewed through the instrument?

Answer 1

Solution:

a)

For light striking the interface from the glass side.

Total internal reflection happens when ray of light travels from denser medium tl rarer medium. Glass is denser than air.

b) Ray a goes up straight through both interfaces.

When the angle of incidence is zero degree, the ray travels undeviated.

c) Ray b has an incident angle 30 degree at first interface. Let this angle be Let the angle of refraction be r.

By snell's law,

This angle r is equal to the angle of incidence at 2nd interface(glass-air interface). let the refracted angle be .

By snell's law.

Thus, . So angle of refràction at glass/air interface in the gap is equal to the incident angle at first interface.

So,

d] incident angle is 30 degree.

By snell's law,

The angle is less than 30 degrees.