13. LIGHT

Have you ever thought how we see various objects? You may say that eyes see the objects. But, can you see an object in the dark? It means that eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. The light may have been emitted by the object, or may have been reflected by it.

To see an object, light needs to reach our eyes. It can come directly from the source (like the sun) or bounce off the object (like a tree reflecting sunlight). If there's no light, like in a dark room, we can't see anything at all, emphasizing that the eyes alone are not enough; they require light.

Think about a flashlight. In a dark room, without it, you can’t see your toys. But when you turn it on, the light illuminates the room, revealing everything in its path. This demonstrates that your eyes need light to perceive what’s around you.

Do you see any relation between the angle of incidence and the angle of reflection? Are they approximately equal? If the experiment is carried out carefully, it is seen that the angle of incidence is always equal to the angle of reflection. This is one of the laws of reflection.

In reflection, when light hits a surface, the angle it makes with a perpendicular line (normal) drawn to that surface is known as the angle of incidence. The light reflects off the surface at the same angle, which is the angle of reflection. This makes both angles equal, and it's a fundamental principle of how light behaves.

Imagine throwing a ball at a wall. If you throw it at a 30-degree angle, it bounces back at the same 30 degrees. Just like this, light follows the same principle, reinforcing the idea that it behaves predictably.

Since the reflected rays do not actually meet at I, but only appear to do so, we say that a virtual image of the point O is formed at I. As you have learnt already in Class VII, such an image cannot be obtained on a screen.

When you look into a mirror, it appears that you can see a reflection of the object behind it, but the light rays don’t actually come from a point behind the mirror. Instead, they create a virtual image. This image can’t be captured on a screen because it’s not real—it just looks like it is.

Think about how you see your reflection when you're dressing up. You can see your outfit, but you can't touch the 'image' of yourself that appears behind the mirror because it’s just an illusion created by how light bounces off the mirror.

When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection. Remember that the diffused reflection is not due to the failure of the laws of reflection.

Reflection can be categorized as regular or diffused based on the surface. Regular reflection occurs on smooth surfaces like mirrors, creating clear images, while diffused reflection happens on rough surfaces, scattering light in various directions without producing a clear image.

Picture the difference between looking into a calm lake (regular reflection) and a rippled puddle (diffused reflection). The lake gives you a clear reflection of your face, while the puddle just makes a blurred image because of its uneven surface.

Nearly everything you see around is seen due to reflected light. Moon, for example, receives light from the Sun and reflects it. The objects which shine in the light of other objects are called illuminated objects.

We can categorize light sources into two types: luminous, which produce their own light (like the sun, light bulbs), and illuminated, which don't produce light but can be seen because they reflect light from luminous sources (like the moon). Understanding this helps us clarify why we see some objects and why others are visible only under certain conditions.

Think of a flashlight shining on a soccer ball at night. The ball isn’t luminous because it doesn’t create light on its own; it just reflects the flashlight’s light, allowing you to see it. This is the difference between something that shines by itself and something that only shines because it's lit up.

The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of a second. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.

Our eyes don't immediately forget what they see. This persistence on the retina allows us to perceive motion smoothly, rather than as a series of still images. If something moves quickly enough, like flickering lights, our brains join the images together to create the illusion of motion.

Think of an old movie reel where you see a series of photos shown quickly. If you flip through them fast enough, it looks like the characters are moving! Just like this trick, our eyes process fast-changing visual information similarly, creating the world of motion around us.

--