When a ray of light is incident perpendicularly on a transparent glass slab what will be its angle of deviation?

Refraction is the property of light due to which it bends its path while travelling from one medium to another. The law of refraction is also known as Snell’s law which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to constant. Below is an experiment to trace the path of a light ray passing through the rectangular glass slab.

To trace the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, and angle of emergence, and interpret the result.

Following are the laws of refraction:

The incident ray, the normal at the point of incidence, and the refracted ray lie in the same plane.
Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.

\(\begin{array}{l}\frac{sin\;i}{sin\;r}=constant\end{array} \)

What is refraction of light?

The refraction of light is a property of light due to which it changes its path when it passes from one medium to the other.

What is lateral displacement?

Lateral displacement is defined as the perpendicular shift in the path of light when it emerges out from the refracting medium.

A drawing board
4-6 all pins
White sheet of paper
Rectangular glass slab
A protractor
A scale
A pencil
Thumb pins

Fix a white sheet on the soft drawing board using thumb pins.
Place the glass slab at the centre of the white paper and draw its outline boundary using a sharp pencil.
Let ABCD be the rectangular figure obtained by drawing.
Mark a point E on AB and draw a perpendicular EN and label it as a normal ray.
Draw one angle of 30° with the help of protractor with EN. Fix pins at P and Q at 4-5 cm on the ray that is obtained by the angle.
Place the glass slab on the rectangular figure ABCD.
To fix R and S, see through the glass slab from side CD, such that when seen through the glass slab, all the pins P, Q, R, and S should lie in a straight line.
Draw small circles around the pins P, Q, R and S and remove the pins.
Remove the glass slab.
Join points R and S such that it meets CD at point F. Draw a perpendicular N’M’ to CD at point F.
Using a pencil, join the points E and F.
Measure the angles formed at AB and CD, i.e, the incident angle, refracted angle, and emergent angle.
The lateral displacement is obtained by extending the ray PQ in a dotted line which is parallel to ray FRS.
Measure the lateral displacement.
Repeat the same procedure for angles 45° and 60°.

Sl.no	Angle of incidence ∠i = ∠PEN	Angle of refraction ∠r = ∠MEF	Angle of emergence ∠e = ∠SFM’	∠i – ∠e ∠PEN – ∠SFM’
1	30°	28°	30°	0°
2	45°	43°	44.8°	0.2°
3	60°	56°	59.8°	0.2°

Due to human error the value of ∠i – ∠e may not be equal to zero.

The angle of incidence and the angle of emergence are almost equal.
As the light is traveling from rarer to denser optical medium, the angle of refraction will be lesser than the angle of incidence.
For different angles of incidence, the lateral displacement will remain the same.
The light will bend towards the normal when it travels from an optically rarer medium to an optically denser medium.

The rectangular glass slab used should have perfectly smooth faces.
The drawing board should be soft so that pins can be easily fixed on it.
The angle of incidence should lie between 30° and 60°.
All pins base should be in a straight line.
The distance between the pins P and Q or the pins R and S, about 5 cm gap should be maintained.
Using a sharp pencil, draw thin lines.
The quality of the protractor should be good.
The placement of the protractor should be correct to get correct measurements.
The perpendiculars should be drawn with care.

There shouldn’t be any air bubbles in the glass slab.
The measurements done by the protractor should be accurate.

Q1. How is the angle of incidence related to the angle of emergence?
Ans: The angle of incidence is equal to the angle of emergence or may differ by very small amount due to human error.

Q2. What should be the angle of incidence for the above experiment?
Ans: The angle of incidence required for the above experiment is between 30° to 60°.

Q3. On which condition does the light not deviate during the refraction of light?
Ans: When the light ray is perpendicular to the refracting surface, there is no deviation in the light ray.

Q4. Which property of light causes the rainbow formation?
Ans: The refraction of light causes the rainbow formation due to tiny water droplets in the atmosphere.

Q5. On which condition will the emergent and incident rays be parallel?
Ans: When the angle of incidence is equal to the angle of the emergent ray, the emergence and incident ray will be parallel.

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Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. (By convention, all angles in geometrical optics are measured with respect to the normal to the surface—that is, to a line perpendicular to the surface.) The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection can be used to understand the images produced by plane and curved mirrors. Unlike mirrors, most natural surfaces are rough on the scale of the wavelength of light, and, as a consequence, parallel incident light rays are reflected in many different directions, or diffusely. Diffuse reflection is responsible for the ability to see most illuminated surfaces from any position—rays reach the eyes after reflecting off every portion of the surface.

law of refraction

When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g., air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. As the transmitted light moves into the second medium, it changes its direction of travel; that is, it is refracted. The law of refraction, also known as Snell’s law, describes the relationship between the angle of incidence (θ1) and the angle of refraction (θ2), measured with respect to the normal (“perpendicular line”) to the surface, in mathematical terms: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the index of refraction of the first and second media, respectively. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.

By definition, the index of refraction for a vacuum is exactly 1. Because the speed of light in any transparent medium is always less than the speed of light in a vacuum, the indices of refraction of all media are greater than one, with indices for typical transparent materials between one and two. For example, the index of refraction of air at standard conditions is 1.0003, water is 1.33, and glass is about 1.5.

The basic features of refraction are easily derived from Snell’s law. The amount of bending of a light ray as it crosses a boundary between two media is dictated by the difference in the two indices of refraction. When light passes into a denser medium, the ray is bent toward the normal. Conversely, light emerging obliquely from a denser medium is bent away from the normal. In the special case where the incident beam is perpendicular to the boundary (that is, equal to the normal), there is no change in the direction of the light as it enters the second medium.

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double convex lens

Snell’s law governs the imaging properties of lenses. Light rays passing through a lens are bent at both surfaces of the lens. With proper design of the curvatures of the surfaces, various focusing effects can be realized. For example, rays initially diverging from a point source of light can be redirected by a lens to converge at a point in space, forming a focused image. The optics of the human eye is centred around the focusing properties of the cornea and the crystalline lens. Light rays from distant objects pass through these two components and are focused into a sharp image on the light-sensitive retina. Other optical imaging systems range from simple single-lens applications, such as the magnifying glass, the eyeglass, and the contact lens, to complex configurations of multiple lenses. It is not unusual for a modern camera to have a half dozen or more separate lens elements, chosen to produce specific magnifications, minimize light losses via unwanted reflections, and minimize image distortion caused by lens aberrations.