Grade 8 - Light and Optical Systems

Section 1: What is Light

Light is made of vibrating electric and magnetic energy. This energy travels as a wave, it has both frequency and amplitude. Light waves vibrate in a direction perpendicular to the direction of motion, i.e., transverse waves. Light waves can travel with or withour a medium. In a vacuum, light travels faster, but travels slower in a medium such as water, glass or air. Scientists have not found anything else that travels faster than light, and it is thought that the speed of light may be the speed limit of our universe.

The wavelength is the distance between one peak and the next peak in a wave. When you multiply the wavelength and the frequency, you get the speed of the wave.

Light as a particle

Light behaves both as a wave and also as a particle. light is like a particle in several ways. It travels in straight lines called light rays. Light does not have a mass, like a particle, but has momentum like a particle. When light hits a surface, it acts like a tiny particle. When light hits a camera film, it produces little dots instead of forming an image all at once. Over time, these dots will add up and form the original image. Light particles are called Photons. A photon is a tiny bundle of energy by which light travels. The energy is a single photon is very small. Each photon also acts like a wave, with a frequency. The higher the frequency the higher the energy.

When light hits an object, photons bounce off at random angles. This is called scattering. We see objects because as light scatters off them, it enters our eyes. Sometimes when light hits an object, light photons are absorbed. Darker surfaces absorb more light than lighter surfaces. When light is absorbed by a surface it is converted into thermal energy and the surface feels hot.

Light may also pass through some objects. Objects that allow light to pass through are called transparent. Objects that blur light as it passes through are called translucent. Objects that allow little to no light through are called opaque.

Opaque objects create a distinct shadow. A shadow is just the absence of light.

An image is a picture of the light source that light makes bouncing off a shiny surface. An image in a mirror is clear because most of the light waves reflect the same way off the mirrors smooth surface.

Section 2: Reflection

Laws of reflection

  • The angle of an incoming ray equals the angle of the reflected light ray.
  • An image in a flat mirrow appears to be behind the mirror
  • The distance to the image is equal to the distance the light traveled to the surface of the mirror.

An image is formed in a mirror because light reflects off all points on the object being observed in all directions. The rays that reach your eye appear to be coming from a point behind the mirror. Because your brain knows that light travels in a straight line, it interprets the pattern of light that reaches your eye as an image of an object you are looking at.

Mirrors can also be made with curved surfaces. If they curve in, they are called concave. If they curve out they are called convex. Curved mirrors can form many kinds of images.

Convex mirrors form images that appear much smaller and farther away than the the object - but they can reflect light from a large area, making them useful as security devices.

Concave mirrors form an image that appears to be closer than it actually is and can be useful because it can also reflect light from a large area - side mirrors on automobiles.

Reflectors help to make bicycles and cars visible at night. A reflector is made up of hundreds of tiny, flat reflecting surfaces arranged at 90o angles to one another. These small surfaces are packed side by side to make the reflector. When light strikes the reflector the light bounces off the tiny surfaces and bounces back toward the light source.

Section 3: Refraction

Refraction

Refraction occurs when light waves bend as they pass from one medium to another or differing densities. Light waves entering a denser medium bend to make steeper angle with the surface. Rays leaving a denser medium bend in the opposite direction. Lenses are used in eyeglasses to make objects appear in focus. Convex lenses work like concave mirrors and concave lenses work like convex mirrors. Lenses are also used in cameras, telescopes and microscopes to change the size of the image.

Our eyes see and interpret light waves of different wavelengths as different colors. Visible light waves with longer wavelengths appear red. Visible light waves with shorter wavelengths appear violet. All colors fit between these two extremes. White light, like the kind from the Sun, is actually just a collection of many different wavelengths mixed together.

We can use a prism to refract white light into its component wavelengths. The band of color in a rainbow, or from light passing through a prism, is called a spectrum.

The picture on a color television is made up of red, green, and blue dots of light. All colors can be created by mixing red, green, and blue light in the right amounts and proportions. Red, green and blue are called primary colors. If all three are mixed equally, they produce white light.

When equal parts of red, green, and blue light rays are mixed, they form white light.

Light is a form of electromagnetic radiation. Electromagnetic radiations are made up of electric and magnetic waves that can move through space. There are many forms of electromagnetism nesides visible light, they differ in wavelength and the energy they carry. All considered together make up the electromagnetic spectrum.

The sun can produce all forms of electromagnetic radiations from infrared radiation, visible light and ultraviolet light. solar flares contain all forms of electromagnetic radiation.

Section 4: Lenses and Vision

Lenses play a crucial role in vision, both in the human eye and in optical devices.

Parts of the Human Eye:

Cornea: The outermost part of the eye is the cornea, which acts as a transparent window and helps focus light onto the lens.

Lens: The lens is a transparent, flexible structure behind the iris (colored part of the eye) that changes shape to focus light onto the retina.

Retina: Light focused by the lens forms an image on the retina, a light-sensitive layer at the back of the eye.

Focusing: The lens changes its shape through a process called accommodation to focus on objects at different distances. This process allows for clear vision at various distances.

Refraction: Light entering the eye is refracted (bent) by the cornea and lens to converge and form a focused image on the retina.

A lens is a curved piece of transparent material (glass/plastic). When light rays pass through it, the light is refracted, causing the rays to bend. A double concave lens is thinner and flatter in the middle than the edges. Light passing through the thicker more curved areas of the lens will bend more than light passing through the thinner areas, causing the light to spread out or diverge.

A double convex lens is thicker in the middle than around the edges. This causes the light to come together at a focal point, or converge.

The lens in the human eye is a convex lens, which focuses the light rays entering your eye to a point on your retina (a light sensitive area at the back of the eye). The image you see is formed on the retina. Some people however have eyes that are too long or too short. If their eye is too long, the image forms in front of the retina - this is a condition called Myopia, or near-sightedness

If their eye is too short, the image forms behind the retina, making object that are close to them difficult to see. This condition is called far-sightedness.

Applications of Lenses

Corrective Lenses:
1. Eyeglasses:
Concave lenses are used to correct nearsightedness (myopia), where distant objects appear blurry.
Convex lenses are used to correct farsightedness (hyperopia), where close objects appear blurry. 2. Contact Lenses: Similar to eyeglasses, contact lenses correct refractive errors. They sit directly on the eye's surface.

Optical Devices:

1. Microscopes:
Lenses are fundamental components in microscopes, where they magnify and focus light to observe small objects.
2. Telescopes:
Telescopes use lenses or mirrors to gather and focus light from distant celestial objects, enabling astronomers to observe stars, planets, and galaxies.
3. Cameras:
Camera lenses focus light onto the camera sensor or film, forming images.

Eye and the Camera

In a camera, if an object moves closer to the film, the lens must move away to keep the image in focus. In the human eye, the lens cannot move, so the ciliary muscles change the shape of the lens (by making the lens bulge in the middle if the image comes closer to you and stretch if the object is further away). This is done so that the eyeball isn't stretched. The process of changing the shape of the lens is called accomodation. The shortest distance at which an object is in focus is called the near point of the eye. The longest distance is called the far point of the eye. On average, an adult has a near point of about 25 cm, whereas babies have a near point of only 7 cm.

In order to adjust the amount of light that enters the eye and the camera, a special device opens and closes to let just the right amount of light in. In the camera, the diaphragm controls the aperture (opening) of the lens and the shutter limits the passage of light. In the eye, the device (or part of the eye) that controls the amount of light entering is called the iris (the colored part of the eye), which changes the size of the pupil - in much the same way as the diaphragm controls the aperture (opening) of the camera lens. The natural adjustment in the size of the pupils is called the iris reflex, which is extremely rapid. This iris reflex action automatically adjusts the pupil when you go from a darkened area to a well lit area, or, from a well lit area to a darkened one.

The Film at the back of the camera contains light sensitive chemicals which change when light hits it. These chemicals form the image on the film. In the eye, when the cells in the retina detect light, they produce small electrical impulses from the retina to the brain by way of the optic nerve.

The point where the retina is attached to the optic nerve does not have any light sensitive cells. This point is known as the blind spot.

The parts of a camera are housed in a rigid light-proof box, whereas layers of tissue hold the different parts of the eye together. The eyeball contains fluids, called humours, which prevent the eyeball from collapsing and refract the light that enters the eye.

Section 5: Extending Human Vision

Telescopes

The telescope is a device used to form magnified images of distant objects.

In a refracting telescope, light from a distant object is collected and focused by a convex lens called the objective lens. A second lens, called the eyepiece lens, works as a magnifying glass to enlarge the image.

A reflecting telescope uses a concave mirror to collect rays of light from a distant object. This mirror is called the primary, or objective mirror, which forms a real image magnified by the eyepiece lens.

Binoculars

Binoculars are two reflecting telescopes mounted side by side. In binoculars, the telescopes are shortened by placing prisms inside, which serve as plane mirrors. In this way, the light entering the binoculars can be reflected back and forth inside a short tube.

Microscopes

A mangifying glass is a simple microscope with a low magnification. There are more advanced microscopes with much higher magnification and imaging technologies described earlier in this chapter.

Section 6: The Wave Model of Light

Light waves are a form of electromagnetic radiation that propagate through space. They exhibit both wave-like and particle-like characteristics, known as wave-particle duality. The properties of light waves include wavelength, frequency, amplitude, and speed. The electromagnetic spectrum encompasses the entire range of light waves, from radio waves with long wavelengths to gamma rays with short wavelengths. Light waves can be reflected, refracted, diffracted, and polarized, interacting with various materials and mediums. Understanding light waves is crucial in fields such as optics, astronomy, and communication, and it plays a fundamental role in vision and technology.

Sunsets can be explained using the wave model of light. As light waves from the sun travel through Earth's atmosphere, they strike particles of different sizes, including dust and other elements. The longer wavelengths of the reds and oranges tend to pass around these particles, whereas, the shorter wavelengths of blue and violet, strike the particles and reflect and scatter.

Laser light, or simply laser (Light Amplification by Stimulated Emission of Radiation), is a type of coherent and focused light that has unique properties, making it useful in a wide range of applications.

Applications of laser

Lasers are used in several applications as follows:

  • Medical Uses: Including surgery, dentistry and medical imaging.
    Surgery: Lasers are used for precision surgeries, including eye surgeries and skin treatments.
    Dentistry: Lasers are employed in dental procedures for cutting, drilling, and tissue modification.
    Medical Imaging: Lasers contribute to imaging techniques such as laser-induced fluorescence and holography.
  • Communication:
    Fiber Optics: Lasers are crucial for transmitting data through fiber optic cables, enabling high-speed and long-distance communication.
    Free-Space Communication: Lasers can be used for free-space optical communication, especially in space and satellite communication.
  • Manufacturing and Material Processing: including cutting and welding, marking and engraving, 3D Printing etc.
  • Entertainment such as laser light shows.
  • Research and Scientific Applications: Including Spectroscopyand laser cooling.
  • Defense and Security: including Lidar Systems (light detection and ranging).
  • Consumer Electronics: including CD and DVD Players and Barcode Scanners.
  • Environmental Monitoring such as Atmospheric Sensing.

Section 7: Beyond Light

Infrared

Their wavelengths are in millimeters. They are produced by vibrating molecules and atoms (heat) and lasers. Infrared wavelengths have been used extensively for military and civilian applications including target acquisition, surveillance, night vision, homing, and tracking. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

Ultraviolet

electromagnetic waves with a wavelength less than 400nm long. Produced by electrons dropping from very high energy levels to low energy levels. They are used for sterilizing materials, tanning, and generally in research. Ionizing radiations can knock electrons out of atoms and break bonds.

X- Rays

X-rays are produced by a sudden deceleration of very high-speed electrons. They are used in medical and industrial imaging. They are also ionizing radiations, this explains why X-rays can result in cancer, when they break bonds in DNA causing mutations or DNA damage.

Gamma Rays

Gamma rays are produced the decomposition of nuclei either spontaneously or by decelerating atomic nuclei. They are produced in nuclear reactions. They can penetrate matter very deep and can destroy carcinogenic or mutant cells hence they can be used in cancer treatment. They are ionizing radiations and can cause radiation sickness.

Radiation and Life

Radiation is energy travelling through space. Sunshine is one of the most familiar forms of radiation. There would be no life on earth without lots of sunlight, but we have increasingly recognised that too much of it on our persons is not a good thing. In fact it may be dangerous. so we control our exposure to it. Sunshine consists of radiation in a range of wavelengths from long-wave infra-red to shorter wavelength ultraviolet. Beyond ultraviolet are higher energy kinds of radiation which arc used in medicine and which we all get in low doses from space, from the air, and from the earth. Collectively we can refer to these kinds of radiation as lonising radiation. It can cause damage to matter, particularly living tissue. At high levels it is therefore dangerous, so it is necessary to control our exposure.

Radiation in the Environment

Natural radiation reaches earth from outer space and continuously radiates from the rocks, soil, and water on the earth. Background radiation is that which is naturally and inevitably present in our environment. Levels of this can vary greatly. A lot of our natural exposure is due to radon, a gas which seeps from the earth's crust and is present in the air we breathe.