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The refractive index (or refraction index) of an optical medium is a dimensionless quantity that indicates the ability of the medium to bend light.
- The refractive index indicates how much light is bent, or refracted, as it enters a material.
- In optics, a lens is a piece of glass or another transparent material that is used to produce an image of an object by focusing light rays from the object.
- The convex lens is a lens that converges light rays that are parallel to its principal axis and is relatively thick in the center and thin at the lower and upper edges.
- A plane mirror is a mirror with a flat or highly polished surface that reflects light to produce an image.
If u is the distance of the object, and v is the distance of the image from the lens, then according to the Lens formula, the focal length of the lens can be given by
1/f = 1/v – 1/u
| Table of Content |
Key Terms: Refractive index, Plane mirror, Convex lens, Lens maker’s formula, focal length, Radius of curvature, Spherometer.
Aim
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To find the refractive index of a liquid using a convex lens and plane mirror.
Apparatus Required
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- Plane Mirror
- Convex Lens
- Optical Needle
- Plane glass slab
- An iron stand with base and clamp arrangement
- The clean transparent liquid in a clean beaker
- Plumb Line
- Half metre scale
- Spherometer
Theory
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If f1 and f2 are the focal lengths of the glass convex lens and liquid lens, respectively, and F is the focal length of their combination, then
1/F = 1/f1 + 1/f2
From the Len’s Maker Formula, we have
1/f2 = (ɳ - 1) [1/R1 - 1/R2]
Where ɳ is the refractive index.
On simplifying the above equation, we get
1/f2 = (ɳ - 1)/R
On substituting the value of f2, the value of refractive index ɳ can be calculated.
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Diagram
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Diagram
Procedure
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To measure the focal length of the convex lens
- Choose a convex lens and estimate its approximate focal length.
- Place the plane mirror on the iron stand's horizontal base.
- Connect the convex lens to the plane mirror.
- Screw the optical needle into the stand's clamp and place it horizontally above the lens at a distance equal to its approximate focal length.
- Bring the tip of the needle to the lens's principal vertical axis, so that it seems to be touching the tip of its image.
- Move the needle up and down to remove the parallax between the needle's tips and its image.
- Using a plumb line and a half-meter scale, measure the distance between the tip of the lens and the upper surface.
- Measure the distance between the tip and the surface of the plane mirror as well.
To measure the focal length of the combination
- Place a few droplets of transparent liquid on the plane mirror and cover it with a convex mirror.
- Repeat steps 6, 7, and 8.
- Make a note of your observations.
To measure the radius of curvature of the convex lens surface
Observations
The rough focal length of convex lens = _____ cm
| Arrangement | The distance of Needle Tip | Focal Length x (cm) | ||
|---|---|---|---|---|
| From lens surface x1 (cm) | From plane mirror x2 (cm) | Mean x = (x1 + x2) cm | ||
| 1 | (2a) | (2b) | (2c) | (3) |
| Without Liquid | f1 = ……… | |||
| With Liquid | F = ……… | |||
The radius of curvature of the convex lens surface, R = _____ cm.
Calculations
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To calculate the value of f2, the formula given below can used
1/f2 = 1/F - 1/f1
With the help of f2 and R, the value of refractive index ɳ can be calculated
ɳ = 1 + R/f2
Precautions
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The following precautions should be taken while performing the experiment
- To avoid thickening the coating, only a few drops of liquid should be used.
- The parallax should be removed from tip to tip.
- The liquid used should be transparent.
Sources of Error
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The sources of error can be observed while performing the experiment if
- The liquid may not be completely transparent.
- The parallax may not be completely eliminated.
Also Read:
| Important Topics | ||
|---|---|---|
| Uses of Concave Mirror | Angle of Incidence | Derivation of Prism Formula |
| Optics | Mirror Formula | Periscope |
| Luminance | Fisheye Lens | Toric Lens |
Things to Remember
- The aim of this experiment is to find the refractive index of a liquid using a convex lens and plane mirror.
- plane Mirror, convex Lens, optical needle, plane glass slab, an iron stand with base and clamp arrangement, the clean transparent liquid in a clean beaker, plumb Line, half meter scale, and spherometer
- The convex lens, which is relatively thick in the center and thin at the bottom and upper edges, is a lens that converges light rays that are parallel to its principal axis
- A mirror with a flat reflective surface is known as a plane mirror.
- The focal length of a plane mirror is infinity.
- Lens maker’s formula is given by 1/f2 = (ɳ - 1) [1/R1 - 1/R2]
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Sample Questions
Ques. What is the Refractive Index of a medium? (2 marks)
Ans. The refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. It is a dimensionless number that explains how light moves through a material.
Ques. Define lens. (1 mark)
Ans. A lens is a piece of transparent material that, by refraction, focuses or disperses light beams.
Ques. What is a convex lens? (2 marks)
Ans. The convex lens is a lens that converges light rays that are parallel to its principal axis (i.e. converges incoming rays towards the principal axis) and is relatively thick in the center and thin at the lower and upper edges.
Ques. What is a concave lens? (1 mark)
Ans. Concave lenses are thick along the edges and thin in the center. These lenses have the ability to diverge light that falls on their surface. As a result, they are also known as diverging lenses.
Ques. What are the factors affecting the refractive index of a medium? (2 marks)
Ans. The following factors affect the refractive index of a medium
- the nature of the material
- the wavelength of light used
Ques. What are the laws of reflection? (2 marks)
Ans. According to the laws of reflection
- The incidence angle is always equal to the reflection angle.
- The incidence angle, refraction angle, and normal to the point of incidence all lie in the same plane.
Ques. Can the refractive index of a medium be less than or equal to 1? (2 marks)
Ans. Because the speed of light in any medium is less than that in a vacuum, the Refractive index of a transparent material is always greater than one.
Ques. Define plane mirror. (2 marks)
Ans. A plane mirror is one that has a flat reflective surface. The angle of reflection equals the angle of incidence for light rays striking the plane mirror. The incidence angle is the angle formed by the incident ray and the surface normal.
Ques. Which substance has the greatest refractive index? (1 mark)
Ans. Diamond has the greatest value of refractive index i.e. 2.42.
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