Refraction of Light:
When a ray of light moves from one medium to another, like from air to water or from air to glass, it changes direction. This change in direction is called refraction of light.
Explanation:
Entering a Denser Medium (e.g., Water to Glass): The light ray bends towards the normal line, which is an imaginary line drawn perpendicular to the surface where the light enters the new medium.
Entering a Rarer Medium (e.g., Glass to Air): The light ray bends away from the normal line.
If the Light Ray is Parallel to the Normal: When a ray of light hits the boundary between two mediums at an angle parallel to the normal line, it goes straight through without bending.
Law of Refraction:
Incident Ray, Refracted Ray, and Normal in the Same Plane: When light moves from one medium to another, like from air to water, it bends. The law states that the incoming ray of light (incident ray), the bent ray of light (refracted ray), and an imaginary line perpendicular to the surface where the light hits (normal) all lie in the same flat plane.
Snell's Law: This law explains how much light bends when it moves from one medium to another. For a specific color of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction (sin(θi)/sin(θr)) equals a constant value represented by the symbol μ (mu), which is called the refractive index of the material the light is entering.
(ii) When you look at stars in the sky, especially on a clear night, you might notice that they seem to twinkle. This twinkling happens because of the way light from the stars bends and moves through the Earth's atmosphere.
(iii) Have you ever noticed that the sun looks more oval-shaped when it's rising or setting? This happens because the light from the sun is passing through more of Earth's atmosphere, causing it to bend and appear stretched out.
(iv) If you've ever looked at something underwater from above the surface, like a fish in a pond or a coin at the bottom of a pool, it might seem like it's not where it actually is. This is because light bends as it moves from water to air, making objects appear shifted or closer than they really are. For example, a fish might seem higher up in the water than it actually is when seen from above, or a coin at the bottom of a glass of water might look like it's floating slightly above the bottom.
Critical angle: When light moves from a denser material to a less dense one, the critical angle is the angle at which light bends so much that it travels along the surface of the denser material, with no refraction into the less dense material.
Total Internal Reflection: When light travels from a denser material to a less dense one and the angle at which it hits the boundary is greater than the critical angle, all the light is reflected back into the denser material, without any being transmitted into the less dense material.
Illustrations of total internal reflection
(i) Sparkling of diamond: When light enters a diamond, it bends and reflects internally, creating a sparkling effect.
(ii) Mirage and looming: In hot conditions, light bends as it passes through layers of air of different temperatures, creating illusions like mirages and looming objects.
(iii) Shining of air bubble in water: Light entering an air bubble in water reflects back due to total internal reflection, making the bubble shine.
(iv) Increase in duration of sun’s visibility: Sometimes, the sun can be seen even before it rises or after it sets because light from it undergoes total internal reflection in the Earth's atmosphere.
(v) Shining of a smoked or metal ball in water: When a smoked or metal ball is placed in water, light reflects off its surface and undergoes total internal reflection inside the water, making it shine.
(vi) Optical Fibre: Light travels through optical fibers, which are made of glass or quartz, by repeatedly reflecting off the inner surface due to total internal reflection, even around curves, without losing much energy.
Applications
(i) Transmitting optical signals and pictures: Optical fibers are used to carry light signals, which can encode data like pictures, over long distances with minimal loss, making them crucial for telecommunications and internet.
(ii) Transmitting electrical signals using light: Electrical signals can be converted into light signals and transmitted through optical fibers, allowing for high-speed and efficient communication in technologies like fiber optic communication networks.
(iii) Visualizing internal body sites with endoscopy: Doctors use endoscopes equipped with optical fibers to see inside the body without invasive surgery. Light transmitted through the fibers illuminates internal organs, providing a clear view for diagnosis and treatment.
Refraction of Light Through Lens
A lens is a piece of clear material with curved surfaces. There are two main types: convex and concave.
Convex Lens: This lens bulges in the middle and makes parallel light rays come together. It's also called a converging lens.
Concave Lens: This lens is thinner in the middle and makes parallel light rays spread apart. It's also called a diverging lens.
Some terms regarding a lens:
Focal Point: The point where light rays meet or appear to come from after passing through the lens.
Focal Length: The distance from the centre of the lens to its focal point.
The power of a lens is its ability to bend light. It's measured as the inverse of its focal length in meters, written as P = 1/f, with diopters (D) as its unit.
For convex lenses, the power is positive, and for concave lenses, it's negative.
When you put two lenses together, their powers add up.
Change in the power of a lens:
When a lens is put in a liquid, its focal length and power can change. Here are the possibilities:
If the lens (with refractive index μ) is dipped in a liquid with a smaller refractive index (μ'), like putting a glass lens (μ = 1.5) in water (μ' = 1.33), the lens's focal length increases, so its power decreases.
If the lens is put in a liquid with the same refractive index, the focal length becomes infinite, meaning the power becomes zero. The lens and liquid act like a single medium.
If the lens is in a liquid with a higher refractive index, its focal length increases, and its power decreases. The lens can even change its behaviour, like a convex lens acting like a concave lens. For instance, an air bubble in water seems convex but acts like a concave lens. Also, a glass convex lens (μ = 1.5) in carbon disulphide (μ' = 1.68) behaves like a concave lens.
Formation of Images by Convex Lens
| Position of Object | Position of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At infinity | At Focus | Highly diminished | Real, inverted |
| Beyond 2F | Between F and 2F | Diminished | Real, inverted |
| At 2F | At 2F | Of same size | Real, inverted |
| Between F and 2F | Beyond 2F | Enlarged | Real, inverted |
| At F | At infinity | Highly enlarged | Real, inverted |
| Between optical centre and F | On the same side as the object | Enlarged | Virtual and erect |
When white light goes through a prism, it breaks into different colours, creating what we call a spectrum. This is called dispersion of light.
Different colors bend by different amounts when they pass through the prism. Violet bends the most, while red bends the least. The order of colors in the spectrum is violet, indigo, blue, green, yellow, orange, and red, which you can remember with the acronym VIBGYOR.
This happens because each color travels at a slightly different speed in the prism. The speed is fastest for red and slowest for violet. And because of this, each color experiences a different amount of bending.
In simpler terms, when white light passes through a prism, it splits into its rainbow colours because each colour moves at a different speed, causing them to spread out.
Rainbow:
A rainbow is a beautiful arc of colors that appears in the sky when it's sunny and there's a little bit of rain. It shows up opposite the sun. Rainbows happen because sunlight gets split, or dispersed, and bent, or refracted, by tiny water droplets in the air.
There are two main types of rainbows:
Primary Rainbow: This one forms from sunlight bending once inside a raindrop and then bouncing off the back of the drop before coming out. In a primary rainbow, red is on the outside and violet is on the inside. It looks like it's about 2 degrees wide, with the top part of the rainbow being about 41 degrees above the horizon.
Secondary Rainbow: This rainbow forms from sunlight bending twice inside a raindrop and bouncing off twice before coming out. The colors are in the opposite order compared to a primary rainbow. It's wider, about 3.5 degrees, and appears higher in the sky, around 52.75 degrees above the horizon. It's not as bright as the primary rainbow.
So, primary rainbows are the usual ones we see, while secondary rainbows are rarer and less intense.
Theory of Colors: Color is what we see when light stimulates the rods in our eyes.
Primary Colors: Blue, green, and red are primary colors because all other colors can be made by mixing them in the right amounts. Mixing blue, green, and red together makes white.
Secondary Colors: When you mix two primary colors, you get a secondary color. The three secondary colors are yellow, magenta, and cyan. For example, mixing green and red makes yellow.
When you mix the three secondary colors together, you get white: yellow + magenta + cyan = white.
Complementary Colors: Colors that, when combined, make white light are complementary. A secondary color and the primary color that wasn't used to make it are complementary. For example, red and cyan are complementary, as are blue and yellow, and green and magenta.
In a nutshell, colored television uses the three primary colors.
Color of Objects: An object's color comes from the light it reflects or lets through. If something reflects all the light, it looks white. If it absorbs all the light, it looks black. That's why a red rose looks red in white or red light, but it looks black in blue or green light.
You can understand how an object looks in different colored light with this table:
Object Colors in Different Lights
| Name of Object | In White Light | In Red Light | In Green Light | In Yellow Light | In Blue Light |
|---|---|---|---|---|---|
| White Paper | White | Red | Green | Yellow | Blue |
| Red Paper | Red | Red | Black | Black | Black |
| Green Paper | Green | Black | Green | Black | Black |
| Yellow Paper | Yellow | Black | Black | Yellow | Black |
| Blue Paper | Blue | Black | Black | Black | Blue |
Light Scattering: When light waves hit tiny particles like dust or water droplets, they bounce off in all directions. This is called light scattering.
Scattering is strongest for violet light and weakest for red light.
The blue color of the sky happens because of light scattering.
And when the sun rises or sets, its red color comes from light scattering too.
Interference of Light: When two light waves with the same frequency and a constant phase difference come together and overlap, their combined intensity in some areas changes. This change in intensity is called interference of light.
There are two types of interference:
- Constructive Interference: When the two waves meet with the same phase at certain points, the resulting intensity is at its highest. This is constructive interference.
- Destructive Interference: When the two waves meet with opposite phases at certain points, the resulting intensity is at its lowest. This is destructive interference.
Light Diffraction: When light waves encounter a small obstacle or aperture that's close in size to the wavelength of light, they don't travel in straight lines anymore. Instead, they spread out beyond the area of shadow that would be expected. This spreading of light energy beyond what's predicted by straight-line propagation is called diffraction.
In simpler terms, diffraction is what happens when a beam of light or other waves spread out after passing through a narrow opening or around an edge.
Polarisation of light:
Polarization is a way to understand that light behaves like a transverse wave. Light, which is an electromagnetic wave, has electric and magnetic fields that vibrate at right angles to each other and to the direction the light travels. Normally, in regular light, the electric field vibrates in all directions perpendicular to the direction of the light wave. Polarization happens when you restrict these vibrations to just one direction in a plane that's perpendicular to the direction of the wave. And when we see light, it's mainly because of these vibrations in the electric field.
Human Eye:
Least distance of distinct vision is 25cm.
Remedy: Glasses with diverging lenses (spread light out).
2. Hyperopia (Long-sightedness):
Causes: Eye ball is too short, lens focuses light behind the retina.
Remedy: Glasses with converging lenses (bring light together).
2. Presbyopia:
Cause: Stiffening of eye muscles.
Remedy: Bifocal glasses (two lenses, one for near, one for far).
3. Astigmatism:
Problem: Blurred vision due to uneven curvature of the eye's surface.
Cause: Irregular shape of the cornea.
Remedy: Glasses with cylindrical lenses (correct uneven focus).
Extra Info:
-The retina has rods (for low-light vision) and cones (for color vision).-Rods help see in dim light, while cones detect colors.
A simple microscope is just a small convex lens. To use it, you put the object you want to see closer to the lens than its focal point.
The magnifying power (M) of the microscope is calculated like this:
Where:
- is the distance between the lens and the object (usually 25 cm).
- is the focal length of the lens.
So basically, the magnifying power is 1 plus the distance between the lens and the object divided by the focal length of the lens.
A compound microscope has two convex lenses arranged in a tube. The lens closest to the object is called the objective, and the lens closer to the eye is called the eyepiece.
The objective lens faces the object and has a smaller aperture than the eyepiece. Both lenses have short focal lengths. This setup boosts the magnifying power of the microscope.
Telescope:
Telescopes help us see faraway things that our eyes alone can't spot. There are different types: astronomical, terrestrial, and Galilean.
Astronomical telescopes have two convex lenses in a tube. The one facing the object is the objective, and the one facing the eye is the eyepiece.
The objective lens is big to gather lots of light from the object. It has a longer focal length than the eyepiece. This setup makes faraway things look closer and clearer.
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