Class 12th PCMB

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.

Objective

To find the focal length of convex mirror using a convex lens.

Material Required

An optical bench with four upright, a knitting needle, a half-meter scale, a convex lens, a convex mirror, a half-meter scale and a mirror holder.

Formula Used

Ray Diagram

Calculation

Result

The focal length of the convex mirror = 23.20

Precautions

• The tips of the needles and the centres of the lens and the mirror should be at the same height.
• The parallax should be removed from tip to tip.
• The distance between the lens and the mirror should be small.
• While adjusting the image needle, the lens and the object needle should not be disturbed.
• The index correction should be carefully recorded and applied.

Sources of Errors

• Parallax was not removed completely.
• The uprights are not perfectly vertical.
• Optical centre, pole of the mirror and the tip of the object needle are not placed in a line.

Objective

To find the value of v for different values of u in case of a concave mirror and to find the focal length.

Apparatus Required

A knitting needle, a half metre scale, an opticle bench with three uprights, two needles and concave mirror with mirror holder.

Formula Used

Ray Diagram

Observations

Calculation

Result

Mean value of focal length, f = 15.0 cm

(as per sign convention the focal length of concave mirror is negative) =- 15.0 cm

Focal length of concave mirror = 15 cm

Precaution

• Tip to tip parallax should be removed between needle and the image of the object needle.
• Index correction for u and u should be properly determined and must be applied.
• All the uprights should be vertical and principal axis of the mirror should be horizontal.
• To locate the position of the image, the eye should be at least 30 cm away from the needle.
• Tips of the needles and the pole of the mirror should be at the same height as that of the pole of the mirror.

Sources of Errors

• The parallax removal may not be perfect.
• The uprights may not be vertical.