Objective: To look at some of the effects, besides a change of direction, of refraction.
DISCUSSION: Is the path of light reversible? How do fiber optics work? Why do your feet look closer when you look at them through water? All of these are refraction effects. This lab will look at all three and help you answer these questions.
Methods:
Part I. Reversibility of light
Step 1. Place a refraction dish, half-filled with water (+ a pinch of magnesium carbonate) on a piece of polar graph paper that is on a piece of soft cardboard so that the flat side of the refraction dish is in line with the 90o line and scratch on the flat side is in the exact center of the graph paper.
Step 2. Aim a single light ray from a ray box right at the scratch on the flat side of the refraction dish along any line. Temporarily place a pin in the bemas path about 4- 6 cm from dish to "see" the laser beam. Mark on the paper the incoming (incident) ray.
Step 3. Look at the laser beam from above the dish as it goes thorugh the water. Place a pin veritcally about 4-6 cm from the cruved edge of the dish to mark where the laser beam exits the water. Mark on the paper the line coming out (refracted ray) of the rounded side. Also find the reflected ray, it will be on the flat side mark it. Record in incident ray, the refracted ray and the reflected ray angles. This will be the forward direction.
Step 4. Now direct the single ray toward the scratch mark but this time aim it from the rounded side along the outgoing angle you found in step 2. Record the angle of incidence and the angle of refraction. Skip the reflected angle this step. This will be the reverse direction.
| Direction | Incident Angle | Refracted Angle |
| Forward | . | . |
| Back | . | . |
Q1. When you go in the reverse direction does the beam come out at the same angle (or close) as it did when it went in on the forward direction? Is light reversible?
Q2. When light went from air into the water from the flat side it refracted. When it went out from water into air on the rounded side it did not. Why not? (Hint: The dish is a perfect semicircle and the scratch mark is the center of the circle.)
Q3. What relationship do you notice between the incident angle and the reflected angle?
Part II. Index of Refraction
Step 5. Direct a single ray towards the scratch mark from the
flat side. Start at 0o. Mark the incident and refracted rays
on your polar graph paper. Record the angles in a data table as shown below.
Repeat for angles of 20o, 40o, 60o, and
80o.
|
|
|
|
|
|
ratio |
| . | . . | . | . | . | . |
| . | . | . | . | . | . |
| . | . | . | . | . | . |
| . | . | . | . | . | . |
| . | . | . | . | . | . |
Q4. Which ratio remains constant?
Q5. What is the actual index of refraction of water? What is your calculated value? What is your percent relative error?
Q6. What is the speed of light in water according to your calucated value for index of refraction?
Step 6: Using one of the two ray boxes in the room, point a single ray of light into the refraction dish so that it refracts and separtes the colors red and blue. Record the angle of incidence and refraction for red and blue light.
| Color | Angle of Incidence | Angle of Refraction |
| . | . | . |
| . | . | . |
Q7. What is the index of refraction in water for red light? for blue light?
Q8. Recall what you know about the amount of energy and the frequency and wavelength of different colors of light. Give a good explanation as to why different colors travel at different speeds in a medium. Then using that explanation explain why red and blue have different angles of refraction.
Part III. Total internal reflection
Step 7. This time direct a single beam of light into the refraction dish from the rounded side but aim towards the scratch mark on the
flat side. Keep increasing the angle until you can find the exact angle
at which total internal reflection occurs. You will notice total internal
reflection when two things happen. (1) the refracted ray disappears and
(2) the reflected ray suddenly increases greatly in brightness. Record
the angle of incidence at which total internal reflection occurs.
measured critical angle = __________
Q8. Calculate the actual critical angle of water using the actual index of refraction of water. Calculate your percent relative error for your measured critical angle.
Q9. What is one device that uses total internal reflection?