Light-Reflection and Refraction Class 10 || Science|| Chapter 9 Notes

Light - Reflection and Refraction Class 10 ||Science|| Chapter 9 Notes

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1. What is Light?

• Light is a form of energy that enables us to see objects. It travels in straight lines and can be reflected or refracted.
• Luminous objects emit light on their own (e.g., the Sun, a lamp), while non-luminous objects are seen only because they reflect light from other sources.

2. Reflection of Light

Reflection is the bouncing back of light when it hits a smooth surface, like a mirror.

a) Laws of Reflection

There are two basic laws:

1. The angle of incidence is equal to the angle of reflection.
   • If a light ray strikes a surface at an angle of 30°, it will reflect back at the same angle.
2. The incident ray, the reflected ray, and the normal (a perpendicular line to the surface) all lie in the same plane.
   • This means that the light ray, the reflected ray, and the normal all stay in one flat space.

b) Types of Reflection

1. Regular Reflection: Occurs on smooth surfaces (like mirrors), and the reflected rays are parallel to each other, resulting in a clear image.

2. Diffuse Reflection: Occurs on rough surfaces (like paper or a wall), where the reflected rays scatter in different directions, so no clear image is formed.

3. Spherical Mirrors

Spherical mirrors are mirrors with a curved surface. They are of two types:

1. Concave Mirror: The reflecting surface is curved inward, like the inside of a spoon.

2. Convex Mirror: The reflecting surface is curved outward, like the back of a spoon.

a) Key Terms for Spherical Mirrors

• Pole (P): The center of the mirror's surface.
• Center of Curvature (C): The center of the sphere from which the mirror is made.
• Radius of Curvature (R): The radius of the sphere.
• Principal Axis: The line passing through the center of curvature and the pole.
• Focus (F): The point where rays parallel to the principal axis converge (concave) or appear to diverge from (convex).
• Focal Length (f): The distance between the pole and the focus. It is half of the radius of curvature (f = R/2).

b) Image Formation by Concave Mirror

A concave mirror can form both real and virtual images, depending on the position of the object:

• Real image: Formed when the light rays actually meet at a point.
• Virtual image: Formed when the light rays appear to meet at a point.

Here are different cases of image formation:

Object Position	Image Position	Image Nature	Image Size
At infinity	At focus (F)	Real and inverted	Highly diminished
Beyond center (C)	Between F and C	Real and inverted	Diminished
At center (C)	At center (C)	Real and inverted	Same size
Between C and F	Beyond C	Real and inverted	Enlarged
At focus (F)	At infinity	Real and inverted	Highly enlarged
Between F and mirror	Behind the mirror	Virtual and erect	Enlarged

c) Image Formation by Convex Mirror

A convex mirror always forms:

• A virtual, erect, and diminished image.
• Convex mirrors are used in rearview mirrors in vehicles because they provide a wide field of view.

4. Mirror Formula and Magnification

a) Mirror Formula:

$$\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">f</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">v</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">+</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">u</span></span>}}$$

Where:

• f = focal length
• v = image distance
• u = object distance

b) Magnification (m):

$$\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">m</span></span>}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\text{<span style="background-color: white;"><span style="font-family: Kanit; font-size: medium;">height of the image (h₂)</span></span>}}{\text{<span style="background-color: white;"><span style="font-family: Kanit; font-size: medium;">height of the object (h₁)</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">-</span></span>\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">v</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">u</span></span>}}$$

• If m > 1, the image is enlarged.
• If m < 1, the image is diminished.
• Negative magnification indicates an inverted image, while positive magnification indicates an erect image.

5. Refraction of Light

Refraction is the bending of light as it passes from one medium to another of different density (e.g., from air to water).

a) Laws of Refraction:

1. The incident ray, the refracted ray, and the normal all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This is called Snell's Law.
   • $$$$
   \frac{\sin i}{\sin r} = \text{constant} = n ] Where n is the refractive index of the medium.

b) Refractive Index:

• The refractive index (n) is a measure of how much light bends when entering a new medium.

  • For example, the refractive index of water is 1.33, meaning light bends more in water than in air.

  • $$$$
  n = \frac{\text{Speed of light in vacuum (c)}}{\text{Speed of light in the medium (v)}} ]

6. Refraction Through a Glass Slab

When light passes through a rectangular glass slab:

1. It first bends towards the normal when entering the slab (air to glass).
2. It then bends away from the normal when exiting the slab (glass to air).
3. The emergent ray is parallel to the incident ray but is laterally displaced (shifted sideways).

7. Spherical Lenses

Lenses are transparent objects that refract light to converge or diverge rays.

a) Types of Lenses:

1. Convex Lens (Converging Lens): Bulges outward and brings parallel light rays to a focus.
2. Concave Lens (Diverging Lens): Curves inward and spreads out light rays.

b) Terms Related to Lenses:

• Optical center (O): The center of the lens.
• Principal Axis: The line passing through the optical center.
• Focus (F): The point where light rays converge (convex) or appear to diverge from (concave).
• Focal Length (f): The distance between the optical center and the focus.

c) Image Formation by Convex Lenses:

Object Position	Image Position	Image Nature	Image Size
At infinity	At focus (F)	Real and inverted	Highly diminished
Beyond 2F	Between F and 2F	Real and inverted	Diminished
At 2F	At 2F	Real and inverted	Same size
Between F and 2F	Beyond 2F	Real and inverted	Enlarged
At focus (F)	At infinity	Real and inverted	Highly enlarged
Between F and lens	On the same side	Virtual and erect	Enlarged

d) Image Formation by Concave Lenses:

A concave lens always forms:

• A virtual, erect, and diminished image.
• Used in spectacles for correcting myopia (nearsightedness).

8. Lens Formula and Magnification

a) Lens Formula:

$$\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">f</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">v</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">-</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">u</span></span>}}$$

Where:

• f = focal length
• v = image distance
• u = object distance

b) Magnification (m):

$$\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">m</span></span>}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\text{<span style="background-color: white;"><span style="font-family: Kanit; font-size: medium;">height of the image (h₂)</span></span>}}{\text{<span style="background-color: white;"><span style="font-family: Kanit; font-size: medium;">height of the object (h₁)</span></span>}}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">v</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">u</span></span>}}$$

9. Power of a Lens

• The power of a lens is a measure of how strongly it converges or diverges light.
• Power (P) is given by:$$\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">P</span></span>}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">=</span></span>\frac{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">1</span></span>}}{\mathrm{<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">f</span></span>}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">(</span></span>\text{<span style="background-color: white;"><span style="font-family: Kanit; font-size: medium;">in meters</span></span>}<span\; style="background-color:\; white;"><span\; style="font-family:\; Kanit;\; font-size:\; medium;">)</span></span>}$$
  • The unit of power is dioptre (D).
  • A convex lens has positive power, while a concave lens has negative power.

10. Conclusion

The study of reflection and refraction helps us understand how light interacts with mirrors, lenses, and different media. These principles are applied in a variety of optical instruments, such as cameras, microscopes, and telescopes.