Theory

Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell’s Law.

ni sin θi=nr sin θr

Where:

θi= angle of incidence

ni=index of refraction of the incident medium

θr= angle of refraction

nr=index of refraction of the refractive medium

Snell’s law is the law of refraction which governs the behaviour of light-rays as they propagate across a sharp interface between two transparent dielectric media.

The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium. Each medium has a different refractive index. The angle between the light ray and the normal as it leaves a medium is called the angle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction. This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.

Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density). This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction.

Image retrieved from http://sciencelearn.org.nz/Contexts/Light-and-Sight/Science-Ideas-and-Concepts/Refraction-of-light

The amount of bending depends on two things. First is the change of speed. If a substance causes the light to speed up or slow down more, it will refract (bend) more. Another thing is the angle of incident ray. If the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

The critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. Note that the critical angle is an angle of incidence value. The actual value of the critical angle is dependent upon the combination of materials present on each side of the boundary.

When a ray light passes through a flat slab of a transparent material, such as a piece of glass plate, the emergent ray is parallel to the incident ray but displaced from it. Consequently, when we look at any object through a glass plate, we see it slightly displaced in position but unchanged. In contrast, lenses use the phenomenon of refraction of light to form images. Concave lens diverge the light incident on it. Thus, it is called the diverging lens. Due to this, these lenses always form diminished, virtual and erect images irrespective of the position of the object in front of them. The magnification produced by these lenses is always less than one. On the other hand, convex lenses converge the light and hence are called the converging lenses. As the object moves away from the lens, the size of its image reduces along with its distance from the lens. Convex lenses form erect, virtual, magnified images or inverted, real, diminished/magnified images depending on the position of the object.

Conclusion:

Based on the data gathered from the experiment, the shallowing effect of refraction was verified. The coin was seen in the same angle as water is added because of its change in refractive index. The index of refraction of glass was determined as 1.4 and it provided 6.7% error. In the second activity, the index of

refraction of glass was calculated to be 1.45 which gives 3.3% error.

References:

Hyperphysics. Refraction. Retrieved on 10/18/15 from http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html

Sciencelearn. Refraction, Retrieved on 10/18/2015 from http://sciencelearn.org.nz/Contexts/Light-and-Sight/Science-Ideas-and-Concepts/Refraction-of-light

Learnnext. Refraction by Spherical Surfaces. Retrieved on 10/18/15 from http://www.learnnext.com/nextgurukul/wiki/concept/CBSE/X/Science/Refraction-by-Spherical-Lenses.htm

PhysicsClassroom. Critical angle. Retrieved on 10/18/15 from http://www.physicsclassroom.com/class/refrn/Lesson-3/The-Critical-Angle