Light - Class 7 Notes & Olympiad Questions

Light

a) Light is a form of energy that allows us to see the world around us.
b) When light hits an object, like a ball or a person, it can bounce off (reflect) or pass through (transmit) it. The object absorbs some of the light and reflects or transmits the rest. Our eyes detect these reflected or transmitted light waves and send signals to our brain, which then interprets them as different colours and shapes.
c) Light travels incredibly fast. It can travel around the Earth more than seven times in just one second!

Rectilinear Propagation of Light

a) Rectilinear propagation of light means that light travels in straight lines. When light travels from a source, like a lamp or the Sun, it moves in a straight path until it hits something or is blocked by something.
b) For example, imagine you have a laser pointer. When you turn it on, the light comes out in a straight line. It doesn't bend or curve on its own. It keeps going straight until it hits an object or a surface. This straight path that light follows is called rectilinear propagation.
c) Understanding rectilinear propagation helps us explain how we see things. When light hits an object, it can bounce off the surface and go into our eyes, allowing us to see the object. If light didn't travel in straight lines, it would be much harder for us to see things clearly.

Reflection of Light

When light hits a smooth and polished surface, like a mirror, it can bounce back. This bouncing back of the light is called reflection. It's the reason why we can see our own image in a mirror.

I. Laws of Reflection

1. The First Law of Reflection

a) The incident ray, the reflected ray, and the normal all lie in the same plane.
b) The incident ray is the incoming ray of light, the reflected ray is the ray of light that bounces off the surface, and the normal is an imaginary line perpendicular (or at a right angle) to the surface at the point of incidence (where the light hits the surface).

2. The Second Law of Reflection

a) The angle of incidence is equal to the angle of reflection. The angle of incidence is the angle between the incident ray and the normal, while the angle of reflection is the angle between the reflected ray and the normal. This law states that the angles of incidence and reflection are always equal.

II. Types of Reflection

1. Regular/Specular Reflection

a) This type of reflection occurs on smooth and shiny surfaces like mirrors. It produces a clear and sharp reflection where the angles of incidence and reflection are equal.
b) Regular reflection allows us to see our reflection with high clarity.

2. Diffused Reflection

a) Unlike regular reflection, diffused reflection happens on rough or uneven surfaces. When light hits these surfaces, it scatters in many different directions. This leads to a hazy or blurred reflection.
b) Diffused reflection is why we can see non-shiny objects, like a book or a wall, even though they don't produce clear reflections like a mirror.

Mirrors and Lenses

I. Plane Mirrors

a) A plane mirror is a type of mirror that has a flat, smooth surface. The mirror is made by coating a thin layer of reflective material, such as aluminium or silver, onto one side of a flat piece of glass.
b) When you look into a plane mirror, it reflects the light that falls on it, allowing you to see yourself or objects in front of it.
c) Plane mirrors have various practical uses. They are commonly used for personal grooming, such as combing hair, applying makeup, or checking your appearance. They are also used in optical instruments, such as periscopes and kaleidoscopes, and in scientific experiments to study the reflection of light.
d) One of the interesting characteristics of a plane mirror is that it forms a virtual image.

Differences Between a Real Image and a Virtual Image

Real Image

a) A real image is an image that can be projected onto a screen or a piece of paper. It is formed when light rays actually come together at a specific point after passing through a lens or reflecting off a mirror.
b) In other words, a real image can be captured and seen outside of our eyes. If you were to place a screen at the location where a real image is formed, you would see a clear and focused image on that screen.

Virtual Image

a) A virtual image is an optical illusion that appears as if it is behind a mirror or a lens, but it doesn't actually exist in that physical space.
b) Virtual images cannot be captured on a screen or paper because they are created by the way our eyes perceive the light rays.
c) When you look into a plane mirror, the light rays from your body or any objects in front of it bounce off the mirror and reach your eyes. Your brain then interprets these rays as coming from behind the mirror, creating the illusion of an image.

Characteristics of an Image Formed by a Plane Mirror

1. Virtual Image

The image formed by a plane mirror is virtual, which means it's not a real object. It's like an optical illusion created by the reflection of light. When you look at a plane mirror, the image you see appears to be behind the mirror, even though there's no physical object there.

2. Laterally Inverted

Another characteristic of the image formed by a plane mirror is that it's laterally inverted. This means the left and right sides of the image appear swapped compared to the actual object. For example, if you raise your right hand in front of a plane mirror, the image will show your left hand raised. It's like looking at a mirror reflection where everything seems flipped horizontally.

3. Same Size

The image formed by a plane mirror is the same size as the object being reflected. If you stand in front of a plane mirror, your reflection will have the exact height, width, and proportions of your real self.

4. Upright

The image formed by a plane mirror is always upright, meaning it appears in the same orientation as the object being reflected. For instance, if you hold a book upright in front of a plane mirror, the image of the book will also appear upright.

5. Formed at the Same Distance

The image formed by a plane mirror is located at the same distance behind the mirror as the object in front of it. This means that the distance between the mirror and the object is the same as the distance between the mirror and the image. So, if you stand 1 meter away from a plane mirror, your reflection will appear to be 1 meter behind the mirror.

Formation of Multiple Reflections by Plane Mirrors

a) When light reflects off a surface and forms an image, that image can act as a new object for reflection on another surface. This process is known as multiple reflection or successive reflections.
b) For example, if you stand in front of a mirror and see your reflection, that reflected image can bounce off another mirror or a reflective surface. The second mirror would then reflect the reflected image, resulting in a cascade of reflections.
c) Each reflection produces a new image, and if there are multiple reflective surfaces arranged in a way that allows the reflections to bounce back and forth, you can see a series of repeated images extending into the distance

a) Parallel Mirrors

When two plane mirrors are placed parallel to each other, no matter how much they are tilted, they will always create an infinite number of images. This effect is known as the “infinity mirror.” This is because the light reflects back and forth between the mirrors, creating a series of reflections that seem to extend indefinitely. Each reflection appears slightly dimmer than the previous one due to the loss of light energy with each bounce.

b) Non-Parallel Mirrors

When two plane mirrors are inclined at an angle to each other, the number of images formed depends on the angle between the mirrors.

1. If the angle between the mirrors is such that when you divide 360 degrees by that angle (θ), you get an even number, then the formula to find the number of images (n) is:

Example: Let's say the angle between the mirrors is 45^o. When you divide 360 by 45, you get 8, which is an even number. Using the formula:

Number of images (n) = (360 / 45) - 1
n = 8 - 1
n = 7

So, in this case, you would observe 7 images of the object.

2. If the angle between the mirrors is such that when you divide 360^o by that angle (θ), you get an odd number, and the object you're looking at is placed exactly in the middle of that angle, then the formula to find the number of images (n) is:

Example: Let's say the angle between the mirrors is 40^o and the object is placed at the bisector of this angle. When you divide 360 by 40, you get 9, which is an odd number. Using the formula:

Number of images (n) = (360 / 40)
n = 9

So, in this case, you would observe 9 images of the object.

II. Spherical Mirrors

Spherical mirrors are special types of mirrors that have curved surfaces, like the inside of a spoon or the surface of a ball.

There are two main types of spherical mirrors: concave mirrors and convex mirrors.

1. Concave Mirrors

a) A concave mirror is curved inward, like the inside of a spoon.
b) When light rays from an object fall on a concave mirror, they reflect off its surface and come together at a point in front of the mirror. This point is called the "focus” or the “focal point” and is denoted by “F.” If you place a screen or a piece of paper at the focus, you can see a clear and magnified image of the object.
c) For concave mirrors, the focal point (F) is located at half the radius of curvature (F = R/2). The radius of curvature (R) is the distance between the centre of the mirror and its curved surface.

Images Formed by Concave Mirrors

a) If the object is placed beyond the focus, a real and inverted image is formed between the focus and the mirror. This type of image can be projected onto a surface, like when you use a projector.
b) If the object is placed between the focus and the mirror, a virtual and magnified image is formed. This means that the image appears larger but cannot be projected onto a surface.

Applications of Concave Mirrors

1. Reflective Telescopes: Concave mirrors are used in reflecting telescopes where they gather and focus incoming light to create clear and magnified images of distant celestial objects.

2. Car Headlights: Concave mirrors help to create a strong and focused beam of light for improved visibility while driving at night.

3. Makeup and Shaving Mirrors: Their ability to form magnified and inverted images allows for close-up and detailed viewing of the face.

4. Dentistry: In dentistry, concave mirrors help dentists to get a better view of the teeth and oral cavity by reflecting light and focusing it on the desired area.

2. Convex Mirrors

a) A convex mirror is curved outward, like the back of a spoon.
b) When light rays from an object fall on a convex mirror, they reflect off its surface and spread out. This makes objects appear smaller than they actually are.
c) In a convex mirror, the focal point (F) is located behind the mirror's surface.
d) The focal length (f) of a convex mirror is half of its radius of curvature (f = R/2).

Images Formed by Convex Mirrors

a) Convex mirrors always produce virtual, upright, and smaller images, regardless of the position of the object.
b) These images can't be projected onto a surface but are useful for seeing a larger area in the reflection.

Applications of a Convex Mirrors

1. Rear-View Mirrors: Convex mirrors provide a wider field of view, allowing drivers to see a larger area behind their vehicle and reducing blind spots.

2. Security and Surveillance: They can be found in shops, malls, and parking lots to provide a wide-angle view and enhance security by allowing people to see around corners and monitor large areas.

3. Road Safety: Convex mirrors are often installed at intersections, sharp bends, and driveways to improve road safety. They help drivers see oncoming traffic and pedestrians from a wider angle, reducing the risk of accidents.

III. Lenses

a) Lenses are transparent objects that can refract light.
b) Refraction is the bending of light when it passes through a different material, like air to water or glass.
c) When light passes through a lens, it changes direction due to refraction. This helps focus the light and form clear images.

There are two main types of lenses: convex lenses and concave lenses.

1. Concave Lenses

a) A concave lens is thinner at the centre and thicker at the edges.
b) When light passes through a concave lens, it bends outward or diverges. This means that parallel rays of light coming from an object spread out after passing through the lens.
c) They cause light to spread out, making objects appear smaller. They cannot form real images, but they can create virtual and upright images.
d) Concave lenses are often used in devices like eyeglasses for people who have difficulty seeing far-away objects. These lenses help to correct nearsightedness and allow the eyes to focus properly.

2. Convex Lenses

a) A convex lens is thicker at the centre and thinner at the edges, resembling a magnifying glass. They are also called "converging lenses."
b) When light passes through a convex lens, it bends inward or converges. This means that parallel rays of light coming from an object are brought closer together after passing through the lens.
c) This lens usually forms a real image, which means you can project it onto a screen or touch it. The image can also be virtual, smaller, larger, or the same size as the object, depending on how far away the object is from the lens.
d) Convex lenses are commonly used in magnifying glasses and cameras to make objects appear larger and clearer. They are also used as corrective eyeglasses for people with difficulty seeing objects up close.

Dispersion of Light

a) Dispersion of light refers to the splitting of white light into its component colours.
b) When white light passes through certain objects like a prism or a droplet of water, it gets separated into a beautiful rainbow of colours.
c) This happens because the light is made up of different colours, like red, orange, yellow, green, blue, indigo, and violet.
d) These colours have different properties, and when they pass through certain objects like water droplets, they bend or change direction differently. As a result, the light spreads out and we can see the different colours separately.
e) This dispersion of light is the reason why we see rainbows after rainfall or when light passes through water droplets in the air. It also explains why you see different colours when light passes through a glass prism.