CBSE Class 10 Science Notes: Light Reflection and Refraction

Reflection Basics

Reflection is the bouncing back of light when it hits a surface.

Angle of Incidence & Reflection: The angle at which light hits a surface (angle of incidence) is equal to the angle at which it bounces back (angle of reflection).

Regular vs. Diffused Reflection:

• Regular Reflection: Occurs on smooth surfaces (like mirrors), where light rays bounce off parallel to each other, forming a clear image.
• Diffused Reflection: Occurs on rough surfaces, where light rays scatter in various directions, which allows us to see objects from any direction.

Spherical Mirrors: Definitions

Spherical Mirrors are curved mirrors, either reflecting inward (concave) or outward (convex).

Concave Mirror: Reflecting surface is curved inwards.

Convex Mirror: Reflecting surface is curved outwards.

• Centre of Curvature (C): The center of the sphere from which the mirror is a part.
• Principal Axis: The straight line passing through the pole (center of mirror) and the center of curvature.
• Principal Focus (F): The point where parallel rays of light converge (concave) or appear to diverge from (convex) after reflection.
• Focal Length (f): The distance between the pole of the mirror and the principal focus. It is half the radius of curvature ($f = R/2$).

Images Formed by Spherical Mirrors

Real vs. Virtual Images:

• Real Images: Formed where light rays actually converge; can be projected onto a screen.
• Virtual Images: Formed where light rays appear to diverge from; cannot be projected onto a screen.

Erect vs. Inverted Images:

• Erect Images: Upright with respect to the object.
• Inverted Images: Upside down with respect to the object.

Ray Diagrams: Used to determine image characteristics based on object location. Key Cases include object at infinity, beyond C, at C, between C and F, and at F, or in front of the mirror (for concave). Convex mirrors always form virtual, erect, and diminished images.

Mirror Formula & Magnification

Mirror Formula: Relates the object distance (u), image distance (v), and focal length (f): $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$.

Magnification (m): Indicates how much larger or smaller the image is compared to the object. It is the ratio of image height ($h_i$) to object height ($h_o$), or the negative ratio of image distance to object distance: $m = \frac{h_i}{h_o} = -\frac{v}{u}$.

Practical Uses of Mirrors

• Shaving Mirrors: Concave mirrors are used to produce a magnified image of the face.
• Headlights: Concave mirrors reflect light from the bulb, focusing it into a beam.
• Solar Concentrators: Concave mirrors focus sunlight to a point for heating purposes.

Laws of Refraction and Refractive Index

Refraction is the bending of light as it passes from one medium to another.

Laws of Refraction (Snell’s Law):

• The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
• The ratio of the sine of the angle of incidence ($i$) to the sine of the angle of refraction ($r$) is a constant, known as the refractive index ($n$): $\frac{sin(i)}{sin(r)} = n$.

Refractive Index (n): A measure of how much light bends when entering a material. A higher refractive index means light bends more. Refractive index is relative to the speed of light in a vacuum, with the formula: $n = \frac{c}{v}$ where c is speed of light in vacuum and v is speed of light in the material.

Refraction through Spherical Lenses: Definitions

Lenses refract light, bending it to form images.

Convex Lenses (Converging): Thicker in the middle, converge parallel light rays to a point.

Concave Lenses (Diverging): Thinner in the middle, diverge parallel light rays.

• Principal Focus (F): The point where parallel rays of light converge (convex) or appear to diverge from (concave) after refraction. Each lens has two principal foci.
• Focal Length (f): The distance between the optical center of the lens and the principal focus.

Images by Lenses

Real vs. Virtual Images: (Same definitions as mirrors). Convex lenses can form both real and virtual images. Concave lenses always form virtual, erect, and diminished images.

Erect vs. Inverted Images: (Same definitions as mirrors).

Ray Diagrams: Used to determine image characteristics. Key cases for convex lenses: object at infinity, beyond 2F, at 2F, between F and 2F, and at F or between the lens and F. Concave lenses’ ray diagrams are straightforward.

Lens Formula & Magnification

Lens Formula: Relates object distance (u), image distance (v), and focal length (f): $\frac{1}{f} = \frac{1}{v} – \frac{1}{u}$.

Magnification (m): Indicates how much larger or smaller the image is: $m = \frac{h_i}{h_o} = \frac{v}{u}$. Note: Sign conventions are crucial here.

Power of a Lens

Power of a Lens: The ability of a lens to converge or diverge light.

Definition: The reciprocal of the focal length in meters: $P = \frac{1}{f}$.

Unit: Dioptre (D). A lens with a focal length of 1 meter has a power of 1 dioptre.

Calculations: For multiple lenses in contact, the total power is the sum of the individual lens powers: $P_{total} = P_1 + P_2 + …$

Refraction through a Prism

Dispersion: The splitting of white light into its constituent colors (VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a prism. Different colors of light have different wavelengths and refract at different angles.

Formation of Spectrum: A spectrum (rainbow) is formed due to dispersion. The prism separates the colors, because of differences in the angle of refraction.

Scattering of Light

Scattering: The redirection of light in many directions when it encounters particles.

Why the Sky Appears Blue: Blue light is scattered more by air molecules than other colors (Rayleigh scattering), hence the sky appears blue during the day.

Examples:

• Tyndall effect (scattering of light by particles in a colloid)
• Appearance of reddish color at sunrise/sunset (though derivation of the color is not required for syllabus)

Applications of Dispersion and Scattering

• Spectroscopy: Used to analyze the composition of substances by studying their spectra.
• Optical Devices: Prisms and lenses in cameras, telescopes, and other optical instruments utilize dispersion and refraction.
• Visibility Phenomena: Scattering is responsible for the colors of the sky, clouds, and the appearance of the atmosphere. (Sunrise/Sunset color formation derivation excluded).