The objective of this experiment is to verify the Snellís law of refraction by tracking a laser ray through a rectangular slab of glass.
A rectangular slab of glass, a laser pointer, a few sheets of paper, a sharp pencil, a ruler, and a protractor
Refraction is the abrupt change in the direction of light as it changes medium. The reason is the change in the speed of light as of light in vacuum is 300,000 km/s, in water 225,000 km/s, and in clear glass 200,000 km/s. It is the difference in sit changes medium. Different transparent media pose different light transmission properties. For example, the speedpeeds that makes light bend as it enters a different medium.
A good analogy to this optical phenomenon is when a car enters a gravel road from asphalt. If the gravel-asphalt borderline is straight and perpendicular (┴) to the road edge as shown in Fig. 1, the car will continue straight but at a reduced speed due to more friction offered by gravel. If the gravel-asphalt borderline is slanted as shown in Fig. 2, then the car pulls to the side that offers more friction to the tire on that side.
Fig. 1: Front tires face equal frictional forces. Car slows down but travels straight.
Fig. 2 : Front tires face unequal frictional forces. Car slows down and pulls to the right.
Light behaves in a similar manner. When a ray of light is incident perpendicularly on the interface between two transparent media, it enters the new medium without bending. Fig. 3.
When light crosses the interface of two media in a slanted way, bending (breaking) of light or refraction occurs. Fig. 4.
Fig.3 Fig. 4
In physics and engineering, normal line means perpendicular line. For practical reasons, angles of incidence ( i ) and refraction ( r ) are measured with respect to the normal line ( N ). This is clearly shown in Fig.4. Both ( i ) and ( r ) are measured with respect to line NN, the normal to the interface.
The refraction index, n, of a transparent medium is defined as the ratio of speed of light in vacuum to the speed of light in that medium. The formula is
where the speed of light c = 300,000 km/s and v is the speed of light in the desired medium. The refraction indices for water and glass are therefore,
Based on this definition, the refraction index of vacuum is 1 because
Air at normal atmospheric pressure is very dilute and has a refraction index of 1.00 very close to that of vacuum.
n air = 1.00
Snellís Law of Refraction:
The Snellís law simply relates angles i and r to the refraction indices of the two media n1 and n2. It is easy to show that
n1 sin ( i ) = n2 sin ( r )
Example: A ray of light that is making a 42.0˚ angle with water surface enters water from air.
Find the angle of refraction that means the angle through which it enters water.
Also find the angle of deviation ( D ).
n1 = 1.00, n2 = 1.33,
i = 90.0˚ - 42.0˚ = 48.0˚ , r = ?
Using Snellís law results in: n1sin i = n2 sin r,
1.00 sin (48.0˚) = 1.33 sin(r) ; sin(r) = sin (48.0˚) / 1.33
r = 34.0˚ ; D = i - r = 48.0 - 34.0 = 14.0˚.
The ray incident on side AA of the slab making angle i1 with NN does refract and enters the slab through angle r1. See Fig. 5. In glass, it travels to the opposite side BB and becomes incident on the other side through angle i2 and finally refracts back into air through angle r2. Mark two points on the incoming ray and two dots on the outgoing ray (with the sharpened pencil) in order to register their locations. Make sure to mark points 1 and 2 (points of incidence) as well.
Given: naccepted = 1.50 (for regular clear glass)
Measured: Angles i1, r1, i2, and r2
Calculations: Apply the Snellís formula to find n once by i1and r1, and once by i2 and r2.
Comparison of the Results: Calculate a %error on n using the usual %error formula.
Conclusion: To be explained by students
Discussion: To be explained by students