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Objective

To find the refractive index of liquid by using convex lens and plane mirror.

Apparatus required

A convex lens, a plane mirror, optical needle, water, an iron stand with base and meter scale.

Formula Used

The refractive index of a liquid by using a convex lens and plane mirror

R= focal length of convex lens.

F= focal length of compound lens formed by convex lens and plano-concave lens of water.

Diagram

1) Mean of fg = (17.5 + 17.4 )/2 =17.45 ≈17.5cm

2) Mean of F = (26+25.9)/2 = 26 cm

Precautions

1. Only a few drops of liquid should be taken so as not to thicken the layer.

2. The parallax should be removed tip to tip.

3. The liquid taken should be transparent.

Sources of Error

1. The liquid may not be quite transparent.

2. The parallax may not be fully removed.

Objective

To determine the refractive index of glass by using travelling microscope.

Apparatus required

A travelling microscope with vernier scale, a glass slab, white paper, a pen and lycopodium powder.

Formula Used

Diagram

Observations

1. Least count of main scale of microscope = 0.5 mm.

2. Number of division on vernier scale=50

Calculation

Result

The refractive index of the glass slab by using travelling microscope = 1.48

PRECAUTIONS

• Microscope once focussed on the cross-mark, the focussing should not be disturbed throughout the experiment. Only rack and pinion screw should be used to raise the microscope upward.
• Only a thin layer of lycopodium powder should be spread on the top of the slab.
• The microscope tube should be vertical.
• Microscope should not be shaky.
• Eyepiece should be adjusted so that cross-wires are distinctly seen.
• As far as possible, the rack and pinion screw should be moved in one direction only to avoid backlash error.

SOURCES OF ERROR

• The scale used in the microscope might be caliberation property.
• Microscope tube may not be exactly vertical.
• The lycopodium powder layer on the glass slab might be thick.

Objective: 

To determine the angle of minimum deviation for a given prism by plotting a graph between angle of incidence and angle of deviation.

Apparatus Required:

 A drawing board, a sheet of paper, a pencil, a glass triangular prism, pins, a half-meter scale, graph paper and a protractor.

Formula Used:

where,

n is the refractive index of the glass prism.

A is the angle of the prism

D_m is the minimum deviation.

Ray Diagram:

Observations:

The angle of the prism, A = 60º

Table for the angle of incidence, i and angle of deviation δ for a prism

Calculation:

To plot the graph between the angle of incidence ∠i and the angle of deviation ∠D, take ∠i along the x-axis and ∠D along the y-axis. Minimum deviation Dm can be found from the graph which would be corresponding to the lowest point in the graph.

Let the value of minimum deviation, Dm= 37°

then,

Putting the A=60° and Dm =37° in the above equation, we get n= 1.5

Result:

• Angle of minimum deviation, Dm= 37º
• the refractive index of the material of the prism, n= 1.5
• The graph indicates as the angle of incidence increases, the angle of deviation first decreases to attain the minimum value of Dm and then again increases with the angle of incidence.

Precautions:

• 35°- 60° is the angle of incidence that needs to be maintained.
• The placement of the pins must be vertical.
• The placement of two pins should be such that the distance is not more than 10mm.
• To represent incident and emergent rays, arrowheads must be marked.
• The angle of the prism used should be the same for all the observations.

Sources of error:

• The pricks made by the pin might be thick.
• Angles might be wrong while measuring them.