Light Energy Science Complete Chapter ICSE - Class 8 Science

Light Energy

Light energy enables us to see our surroundings and is a form of energy that travels through different media. Its behavior changes depending on the medium, leading to phenomena such as reflection, refraction, and dispersion. Understanding these principles is essential for analyzing image formation and the properties of mirrors and prisms.

Speed of Light in Different Media

The speed of light varies depending on the medium it travels through. It is fastest in gases, slower in liquids, and slowest in solids. This difference is due to the density of the medium, which affects how light propagates.…

Light Energy

Light energy enables us to see our surroundings and is a form of energy that travels through different media. Its behavior changes depending on the medium, leading to phenomena such as reflection, refraction, and dispersion. Understanding these principles is essential for analyzing image formation and the properties of mirrors and prisms.

Speed of Light in Different Media

The speed of light varies depending on the medium it travels through. It is fastest in gases, slower in liquids, and slowest in solids. This difference is due to the density of the medium, which affects how light propagates.

• In air (gas): $3 \times 10^8$ m/s

• In water (liquid): $2.25 \times 10^8$ m/s

• In glass (solid): Slower than in water; exact value not specified

• Light cannot travel through opaque materials (e.g., walls).

• Transparent media allow light to pass and undergo changes in speed.

Reflection and Refraction of Light

Reflection occurs when light strikes a surface and bounces back, while refraction is the bending of light as it passes from one transparent medium to another. The direction and angle of bending depend on the density of the media involved.

• Reflection: Light bounces back from a surface (e.g., plane mirror).
• Refraction: Light bends when moving between media of different densities (e.g., air to water).

Rules for refraction:

• From lighter (less dense) to heavier (more dense) medium: light bends closer to the normal.
• From heavier to lighter medium: light bends away from the normal.
• If light hits the surface perpendicularly, it travels straight without bending.

Terms Related to Refraction

Several terms are used to describe the process of refraction. These include incident ray, refracted ray, refracting surface, normal, angle of incidence, and angle of refraction.

• Incident ray: The ray that falls on the surface.
• Refracted ray: The ray that bends after entering the second medium.
• Refracting surface: The boundary where two media meet.
• Normal: An imaginary perpendicular line at the point of incidence.
• Angle of incidence: The angle between the incident ray and the normal.
• Angle of refraction: The angle between the refracted ray and the normal.

Laws of Refraction

The laws of refraction govern how light behaves when passing between media. The first law states that the incident ray, refracted ray, and normal all lie in the same plane. The second law relates the angles of incidence and refraction through a constant ratio.

• First Law: Incident ray, refracted ray, and normal lie in the same plane.
• Second Law: The ratio $\frac{\sin i}{\sin r}$ is constant for two given media, where $i$ is the angle of incidence and $r$ is the angle of refraction. This ratio is called the refractive index ($\nu$).

Effects of Refraction: Apparent Depth and Sun Phenomena

Refraction causes objects submerged in water to appear at a shallower depth than their actual position. It also explains why the sun appears to rise earlier and set later than its actual position.

• Apparent depth: Objects in water appear closer to the surface due to refraction.
• Sunrise and sunset: The sun appears above the horizon before it actually rises and after it sets, due to bending of sunlight through atmospheric layers.

Atmospheric layers:

• Air is denser near the Earth's surface and becomes rarer (less dense) at higher altitudes.
• Sunlight bends towards the normal when moving from lighter to denser air, causing the apparent shift in sun position.

Prism and Dispersion of Light

A prism is a transparent glass body with a triangular cross-section, bounded by five plane surfaces. When white light passes through a prism, it undergoes refraction twice and splits into seven distinct colors, forming a spectrum.

• Prism: Transparent medium with two parallel triangular surfaces and three rectangular surfaces.

• Dispersion: Splitting of white light into seven colors (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red).

• Spectrum: The band of colors formed after dispersion.

• Newton proved that passing the spectrum through another prism recombines the colors into white light, showing that white light is a mixture of seven colors.

• The prism does not create colors; it separates those already present in white light.

Refractive Index and Color Dependence

The refractive index of a medium varies for each color of light. This difference causes the splitting of white light into its constituent colors during dispersion.

• Refractive index ($\nu$): $\nu = \frac{\text{speed of light in air}}{\text{speed of light in medium}}$
• Highest refractive index: Violet light (slows down the most)
• Lowest refractive index: Red light (slows down the least)
• The difference in refractive indices leads to the separation of colors.

Spherical Mirrors: Types and Properties

Spherical mirrors are formed by cutting a sphere into parts and making one surface reflective. There are two types: concave and convex mirrors, each with distinct properties and uses.

• Concave mirror: Reflecting surface is on the inner side (cave-like).
• Convex mirror: Reflecting surface is on the outer side (bulging outward).

Key parts:

• Pole (P): Midpoint of the mirror.
• Center of curvature (C): Center of the sphere from which the mirror is cut.
• Radius of curvature (R): Distance from pole to center of curvature.
• Principal axis: Straight line passing through pole and center of curvature.
• Principal focus (F): Point on principal axis where parallel rays meet after reflection.
• Focal length (f): Distance between pole and focus; $R = 2f$.

Rules for Image Formation by Spherical Mirrors

Image formation by spherical mirrors follows specific rules based on the path of incident rays. These rules help in constructing ray diagrams and predicting image properties.

• Rule 1: A ray passing through the center of curvature reflects back along its own path.
• Rule 2: A ray parallel to the principal axis reflects through the focus.
• Rule 3: A ray passing through the focus reflects parallel to the principal axis.

These rules apply to both concave and convex mirrors, with appropriate adjustments for the direction and position of the focus.

Types of Images Formed by Mirrors

Images formed by mirrors can be real or virtual, inverted or upright, depending on the mirror type and object position.

• Real image: Formed when reflected rays actually meet at a point; can be obtained on a screen; always inverted.

• Virtual image: Formed when reflected rays appear to meet on extending them backward; cannot be obtained on a screen; always erect and upright.

• Concave mirrors can form both real and virtual images.

• Convex mirrors always form virtual, upright, and diminished images.

Image Formation Cases for Concave and Convex Mirrors

The position of the object relative to the mirror determines the nature, size, and position of the image formed. The following cases are explicitly described:

Concave Mirror:

• Object at infinity: Image at focus, real, inverted, highly diminished.
• Object beyond center of curvature: Image between focus and center, real, inverted, smaller.
• Object at center of curvature: Image at center, real, inverted, same size.
• Object between center and focus: Image beyond center, real, inverted, larger.
• Object at focus: Image at infinity, real, inverted, highly magnified.
• Object between focus and pole: Image behind mirror, virtual, erect, enlarged.

Convex Mirror:

• Object at infinity: Image at focus, virtual, upright, highly diminished.
• Object at any other position: Image between focus and pole, virtual, upright, diminished.

Uses:

• Concave mirrors: Solar cookers, dentist mirrors, torches, shaving mirrors.
• Convex mirrors: Rear-view mirrors in vehicles, security mirrors in ATMs.

Table: Image Formation by Spherical Mirrors