Engineering Physics
For B.Tech Students – Theory + Problems
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1. Newton’s Rings
• Theory
Newton’s Rings are circular interference patterns created by the
interference of light reflected between a convex lens and a glass plate.
When monochromatic light strikes the lens, it forms an air film between the
lens and the glass plate.
The thickness of the air film increases from the center outward, resulting in
alternating dark and bright concentric rings.
• Important Formula
Diameter of nth dark ring : D n= √4nR λ
Where:
- Dn = diameter of nth dark ring
- λ = wavelength of light
- R = radius of curvature of lens
- n = ring number
, ❖ Applications
✓ Determining wavelength of light
✓ Determining refractive index of liquids
✓ Measuring radius of curvature of lenses
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❖ Practice Problems
1. A plano-convex lens with a radius of curvature of 1 m is placed on a glass
plate. If the wavelength of light is 589 nm, calculate the diameter of the 5th
dark ring.
2. In a Newton’s Rings experiment, the diameter of the 10th ring is 4 mm.
Find the wavelength if R = 1.2 m.
3. When water is added between the lens and plate, how does the ring
diameter change?
4. If the wavelength increases, what happens to the ring spacing?
For B.Tech Students – Theory + Problems
---
1. Newton’s Rings
• Theory
Newton’s Rings are circular interference patterns created by the
interference of light reflected between a convex lens and a glass plate.
When monochromatic light strikes the lens, it forms an air film between the
lens and the glass plate.
The thickness of the air film increases from the center outward, resulting in
alternating dark and bright concentric rings.
• Important Formula
Diameter of nth dark ring : D n= √4nR λ
Where:
- Dn = diameter of nth dark ring
- λ = wavelength of light
- R = radius of curvature of lens
- n = ring number
, ❖ Applications
✓ Determining wavelength of light
✓ Determining refractive index of liquids
✓ Measuring radius of curvature of lenses
---
❖ Practice Problems
1. A plano-convex lens with a radius of curvature of 1 m is placed on a glass
plate. If the wavelength of light is 589 nm, calculate the diameter of the 5th
dark ring.
2. In a Newton’s Rings experiment, the diameter of the 10th ring is 4 mm.
Find the wavelength if R = 1.2 m.
3. When water is added between the lens and plate, how does the ring
diameter change?
4. If the wavelength increases, what happens to the ring spacing?
