EXPERIMENT No2. R.I. Of A Liquid By Concave Mirror

 EXPERIMENT No2.   R.I. Of A Liquid By Concave Mirror 

To Determine the Refractive Index of a Liquid Using a Concave Mirror

Aim

To determine the refractive index (μ) of a liquid using a concave mirror.

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Apparatus

Concave mirror, index pin, retort stand, liquid (water/oil), meter scale, dropper.

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Theory

When a thin layer of liquid is poured over the concave mirror, the real focus shifts upward because light rays bend towards the normal while emerging from the liquid.
If:

• f = focal length of concave mirror (without liquid)

• f′ = apparent focal length (with liquid)

Then the refractive index of the liquid is:

$$\mu = \frac{f}{f'}$$

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Procedure

1. Mount the concave mirror on a stand.

2. Place an index pin on the principal axis. Adjust its height so its image coincides with itself (no parallax). Measure the focal length f.

3. Pour a thin uniform layer of liquid on the mirror.

4. Adjust the index pin again until there is no parallax between pin and its image. Measure new focal length f′.

5. Use μ = f / f′ to calculate refractive index.

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Observations

Quantity	Value
Focal length without liquid, f = 17	___ cm
Apparent focal length with liquid, f′	= 15___ cm
Refractive index, μ = f/f′ = 1.13	___

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Result

The refractive index of the given liquid is: 1.13

MCQs 

1. The refractive index of a liquid using a concave mirror is found by comparing:

A. Two radii of curvature
B. Two focal lengths
C. Two incident angles
D. Two object distances
Answer: B

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2. When liquid is poured into a concave mirror, the focus:

A. Moves downward
B. Moves upward
C. Disappears
D. Remains unchanged
Answer: B

When a liquid is poured into a concave mirror, the focal length changes because the liquid acts like a transparent medium with a refractive index greater than air. This effectively reduces the mirror’s power, making the reflected rays converge less strongly.

👉 As a result, the focus moves upward (away from the mirror surface).

✅ Correct Answer: B. Moves upward

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3. The formula for refractive index of the liquid is:

A. μ = f′/f
B. μ = f²/f′
C. μ = f/f′
D. μ = f + f′
Answer: C

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4. The image used in this experiment is:

A. Virtual
B. Real
C. Inverted
D. Both B and C
Answer: D

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5. The upward shift of the image is due to:

A. Reflection
B. Diffraction
C. Refraction
D. Interference
Answer: C

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6. The index pin is adjusted until:

A. It touches the mirror
B. It becomes brightest
C. There is no parallax
D. Its shadow disappears
Answer: C

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7. A concave mirror forms a real focus because it:

A. Diverges rays
B. Converges rays
C. Transmits rays
D. Blocks rays
Answer: B

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8. Apparent focal length f′ is:

A. Greater than f
B. Less than f
C. Equal to f
D. Zero
Answer: B

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9. The liquid layer used is:

A. Thick
B. Thin
C. Frothy
D. Gel-like
Answer: B

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10. Refractive index tells how much:

A. Time light takes
B. Light bends
C. Light reflects

$$\mathrm{D.\; Mirror\; shines}$$

                                                                              Quiz

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