CBSE Class 7 Science: Understanding Light

What is Light? Its Properties and Sources

Light, a fundamental aspect of our world, is a form of energy that enables us to see. Without light, everything would be dark, and we wouldn't be able to perceive colours or shapes. We see objects only when light from them (either emitted or reflected) reaches our eyes. There are various sources of light:

• Natural Sources: The Sun is the most important natural source of light. Other natural sources include stars, fireflies, and lightning.
• Artificial Sources: These are man-made sources like electric bulbs, fluorescent tubes, torch lights, candles, and lasers.

One of the most important properties of light is that it always travels in a straight line. This property is known as Rectilinear Propagation of Light. Think about a beam of light coming through a small opening in a dark room – it forms a straight, visible path. Similarly, when you stand in front of the sun, your body blocks the light, and since light cannot bend around you, a dark area called a shadow is formed behind you, demonstrating its straight-line path. This principle is fundamental to understanding how light interacts with objects and forms images.

Understanding Reflection and Its Laws

When light strikes a surface, it can bounce back. This phenomenon of light bouncing back after striking a surface is called reflection. The quality of reflection depends on the surface. Smooth, polished surfaces like mirrors reflect light clearly, while rough surfaces scatter light.

To understand reflection better, let's look at some key terms:

• Incident Ray: The ray of light that falls on the surface.
• Reflected Ray: The ray of light that bounces back from the surface.
• Normal: An imaginary line drawn perpendicular (at 90 degrees) to the reflecting surface at the point where the incident ray strikes.
• Angle of Incidence (∠i): The angle between the incident ray and the normal.
• Angle of Reflection (∠r): The angle between the reflected ray and the normal.

There are two main laws of reflection:

1. The angle of incidence is always equal to the angle of reflection (∠i = ∠r).
2. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

These laws are crucial for predicting how light behaves when it hits a mirror or any other reflective surface, and they form the basis for understanding how mirrors create images.

How a Plane Mirror Forms an Image

1. Object Placement — Imagine an object, let's say an apple, placed in front of a flat, smooth plane mirror. Light rays are continuously reflecting off the apple in all directions.
2. Light Rays Strike the Mirror — Consider at least two light rays originating from a single point on the apple. These rays travel in straight lines and strike the plane mirror's surface.
3. Reflection According to Laws — Each incident ray reflects off the mirror. The angle of incidence for each ray is equal to its angle of reflection. The reflected rays then travel away from the mirror.
4. Virtual Extension of Reflected Rays — When you look into the mirror, your brain perceives the reflected rays as if they are coming from behind the mirror. If you extend these reflected rays backwards (imaginary lines behind the mirror), they appear to meet at a specific point.
5. Image Formation and Characteristics — The point where the extended reflected rays appear to meet is where the image of that point on the apple is formed. Since the light rays do not actually meet but only appear to meet, the image formed by a plane mirror is virtual. It is also erect (upright, not upside down), the same size as the object, and undergoes lateral inversion (left appears right, and vice-versa). The distance of the image behind the mirror is equal to the distance of the object in front of it.

Exploring Different Types of Mirrors and Lenses

Plane Mirror

A flat, smooth reflecting surface. It forms virtual, erect, same-sized, and laterally inverted images. Commonly used as dressing mirrors at home.

Concave Mirror

A spherical mirror whose reflecting surface is curved inwards. It converges parallel rays of light to a single focal point. It can form both real and virtual images depending on the object's position. Used by dentists, in torches, car headlights, and solar furnaces.

Convex Mirror

A spherical mirror whose reflecting surface is curved outwards. It diverges parallel rays of light. It always forms virtual, erect, and diminished (smaller) images. Used as rearview mirrors in vehicles because they provide a wider field of view, helping drivers see more traffic.

Concave Lens

A lens that is thinner in the middle and thicker at the edges. It diverges parallel rays of light. Always forms virtual, erect, and diminished images. Used to correct myopia (nearsightedness), where distant objects appear blurry.

Convex Lens

A lens that is thicker in the middle and thinner at the edges. It converges parallel rays of light to a single focal point. Can form both real and virtual images. Used in magnifying glasses, cameras, telescopes, and to correct hypermetropia (farsightedness), where near objects appear blurry.

Dispersion of Light: The Colours of a Rainbow

Have you ever wondered how a rainbow forms? It's all thanks to a phenomenon called dispersion of light.

White light, like sunlight, is not just one colour. It is actually made up of seven different colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. We can remember them by the acronym VIBGYOR.

When white light passes through a transparent medium, such as a prism or water droplets in the air, it splits into its constituent seven colours. This splitting of white light into its component colours is called dispersion. Each colour bends at a slightly different angle, causing them to separate and become visible individually.

Rainbows are a beautiful natural example of dispersion. Tiny water droplets suspended in the air after rainfall act like millions of tiny prisms, splitting sunlight into its seven colours, which we then see as a magnificent arc in the sky. This shows us that light is truly full of hidden wonders!

Key Concepts and Common Pitfalls to Avoid

When studying light, clarity on certain definitions and properties is essential for scoring well. Remember that real images can be obtained on a screen (formed by the actual meeting of reflected or refracted rays), while virtual images cannot (formed by the apparent meeting of rays). A plane mirror always forms a virtual image. For plane mirrors, specifically note lateral inversion – your left hand appears as the right hand of your image and vice-versa. Another common mistake is confusing the converging nature of concave mirrors and convex lenses with the diverging nature of convex mirrors and concave lenses. Always associate concave mirrors and convex lenses with bringing light rays together, and convex mirrors and concave lenses with spreading them apart. When asked about ray diagrams, ensure your lines are straight and arrows indicate the direction of light. Practice drawing them!

Practice Questions with Solutions

• Q: What is rectilinear propagation of light? Give one example from daily life. A: Step 1: Define rectilinear propagation. It is the property of light that states light always travels in a straight line. Step 2: Provide an example. A common example is the formation of shadows. When an opaque object blocks the path of light, a dark shadow is formed behind it because light cannot bend around the object. Final answer: Rectilinear propagation is the property of light traveling in straight lines. An everyday example is the formation of shadows.
• Q: An object is placed 5 cm in front of a plane mirror. Describe the characteristics of the image formed and state its distance from the mirror. A: Step 1: Recall the characteristics of an image formed by a plane mirror. The image is virtual, erect, the same size as the object, and laterally inverted. Step 2: Recall the distance property of a plane mirror image. The image is formed as far behind the mirror as the object is in front of it. Step 3: Apply the distance to the given problem. Since the object is 5 cm in front of the mirror, the image will be formed 5 cm behind the mirror. Final answer: The image formed will be virtual, erect, the same size as the object, and laterally inverted. It will be formed 5 cm behind the mirror.
• Q: Identify whether a concave mirror or a convex mirror would be suitable for the following uses: a) As a dentist's mirror b) As a rearview mirror in a car A: Step 1: Analyze the requirement for a dentist's mirror. A dentist needs to see a magnified image of the teeth to examine them properly. Step 2: Determine which mirror forms magnified images. A concave mirror can form magnified images when the object is placed close to it. Step 3: Analyze the requirement for a rearview mirror. A rearview mirror needs to provide a wide field of view to see a larger area of traffic behind the car, and the image should be erect. Step 4: Determine which mirror provides a wide field of view and forms erect images. A convex mirror always forms a virtual, erect, and diminished image, giving a wider field of view. Final answer: a) Concave mirror (for magnified image) b) Convex mirror (for wider field of view and erect image).
• Q: What is dispersion of light? Name the colours of white light in order of their appearance when dispersed. A: Step 1: Define dispersion of light. Dispersion is the phenomenon where white light splits into its constituent colours when it passes through a transparent medium like a prism or water droplets. Step 2: List the colours of white light in order. The seven colours are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. Final answer: Dispersion of light is the splitting of white light into its component colours. The colours, in order, are Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).

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