To find the refractive index of water and oil using a plane mirror and an equiconvex lens and an adjustable object needle Physics Investigatory Project PDF Class 12
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INTRODUCTION
In optics, the refractive index (a.k.a. refraction index) of an optical medium is a dimensionless number that indicates the light-bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted when entering a material. This is described by Snell’s law of refraction, n1 Sinθ1=n2 Sinn2,
Where,
θ1= Angles of incidence
θ2= Angle of refraction,
Respectively of a ray crossing the interface between two media with refractive indices n1 and n2
OBJECTIVE
To find the refractive index of water, oil using a plane mirror and an equiconvex lens and an adjustable object needle.
MATERIAL REQUIRED
- Convex lens
- Plane mirror
- Water
- Oil
- Clamp stand
- An Optical needle
- Plumb line
- Knitting needle
- Half meter Scale
- Glass slab
- Spherometer
THEORY
Please read this part in PDF
PROCEDURE
(a) For focal length of convex lens:
- Find the rough focal length of the convex lens.
- Place a plane mirror on the horizontal base of the iron stand and then a convex lens on the plane mirror.
- Hold the needle in the clamp stand and adjust its position on the stand such that there is no parallax between tip of the needle and its image.
- Measure distance between tip and upper surface of the lens by using a plumb line and half meter scale. Also measure the distance between tip of needle and upper surface of the mirror. Take the mean of the two readings. This means distance will be equal to the focal length of the convex lens (fi).
(b) For focal length of the combination.
- Put a few drops of the water on the plane mirror and put the convex lens over it with its same face above as before. The water spreads in a form of layer and acts like a Plano-concave lens.
- Repeat the steps 3 and 4 to determine the equivalent focal length of the combination.
- Record the observation.
- Repeat the steps 5, 6, 7 for other transparent liquid (oil).
( c ) For radius of curvature of convex lens surface:
- Determine the pitch and the least count of the spherometer.
- Remove the convex lens and dry it completely. Put the spherometer on this lens surface.
- All the three legs of the spherometer should be placed symmetrically on the lens and adjust the central screw tip to touch the surface of the lens.
- Remove the spherometer from the surface of the lens and place on the plane mirror surface and record the reading.
- Repeat the steps 10 and 11 three times.
- Obtain the impressions of the three legs of the spherometer on a paper and mark them and their average distance.
OBSERVATION
- Pitch of the spherometer=1 cm
- Least count of the spherometer=0.01 cm
- Distance between the Legs:
(1)AB=3cm
(2)BC=3 cm
(3)CA=3cm
CALCULATION
- Mean Distance Between two legs
l= (AB+ BC+ CA)/3 = 3cm
- Mean of h = (0.555 +0.6)/3=0.5775 cm
- To find the radius of curvature of the convex lens
R= l^2/6h + h/2 = 2.8861 cm
- Measurement of refractive indices of water and oil
- With water between the convex lens and the plane mirror
μ_1=1+R/f_2 = 1+2.8861/34.7=1.0831
- With oil between the convex lens and the plane mirror
μ_2=1+R/f_3 = 1+2.8861/10=1.2886
RESULT
- The refractive Index of Water is μ_1=1.0831
- The refractive Index of Oil is μ_2=1.2886
PRECAUSTIONS
- The plane mirror should be clean and fully shining surface.
- The liquid taken should be transparent.
- The parallax should be removed tip to tip.
- The eye should be at a distance about 30 cm from the needle while removing the parallax.
- Only few drops of liquid should be taken so that its layer should be thick.
- The legs of the spherometer should be vertical.
- The centre leg of the spherometer should turn in one direction only.
SOURCES OF ERROR
- Liquid may not be quite transparent.
- The parallax may not be fully removed.
- The spherometer legs should be placed symmetrical on the surface of the convex lens.
- The tip of the central screw should not just touch the surface of lens or mirror