OPTICS UNIT 2 TEST TUESDAY NOVEMBER 4

PROPERTIES AND FEATURES OF WAVES A wave is a movement that transfers energy through matter or space. Energy is the ability to apply a push or pull on an object. Crest – The highest point in the wave. Trough – The lowest point in the wave. Rest position – Where the water would be if it were still. Wavelength – The distance from crest to crest or from trough to trough. It is usually measured in meters. Amplitude – The height of a wave crest or depth of a wave trough as measured from its rest position. It is related to the amount of energy carried by the wave.

FREQUENCY FORMULA Frequency – The number of repetitive motions that occur in a given time. It is usually measured in hertz (Hz), or cycles per second. Frequency = cycles ÷ seconds

TRANSVERSE WAVES Matter in the medium moves up and down perpendicular to the direction that the wave travels.

COMPRESSION WAVES Matter in the medium moves back and forth along the same direction that the wave travels.

WHAT IS VISIBLE LIGHT? The wave model of light pictures light travelling as a wave. In this model, light is a wave that travels through empty space and transfers energy from one place to another. Visible light is light that you can see. It includes all the colours of the rainbow. Refraction is the bending of a wave at the border between one medium and another.

REFRACTION OF LIGHT (CONTINUED)

WHAT ARE THE COLOURS OF VISIBLE LIGHT? Visible light is made up of a range of colours. There are 7 categories: Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Different colours are light waves of different wavelengths. Red has the longest wavelength (700 nm) Violet has the shortest (400 nm) Visible Spectrum: the range of colours or frequencies of visible light.

WHY DO OBJECTS APPEAR COLOURED? Reflection occurs when a light wave strikes an object and bounces off. Different materials absorb and reflect different wavelengths of light. You see only the reflected wavelengths. Red ball looks red because it absorbs all wavelengths of visible light except for those around 700 nm.

HOW DO COLOURS OF LIGHT COMBINE?

BEYOND VISIBLE LIGHT & ELECTROMAGNETIC RADIATION Light spreads out (radiates) from the Sun and other stars in all directions. Energy, such as light, that travels by radiation is called radiant energy. Sun also radiates invisible energy. Light we see is a tiny band of a much broader spectrum of energy. Both the visible light waves and invisible waves are known as electromagnetic radiation. The spectrum of electromagnetic radiation is known as the electromagnetic spectrum.

ELECTROMAGNETIC SPECTRUM

THE RAY MODEL OF LIGHT The Ray Model of Light shows the direction of light as it moves in a straight line. Three things happen when light strikes a material: 1.Transmitted (Passes straight through it) 2.Reflected (Bounces off it) 3.Absorbed (Becomes “trapped” in it)

WHAT CAN HAPPEN WHEN LIGHT STRIKES DIFFERENT MATERIALS? 1.If all or most of the light is transmitted, the material is transparent. 2.If all or most of the light is reflected or absorbed so that none of the light passes through, the material is opaque. 3.If only some of the light is transmitted, and that light is scattered in all directions, the material is translucent.

SHADOWS The ray diagram, can also be used to show how the size of shadows is related to the distance of the object from the light source.

WHAT HAPPENS WHEN LIGHT REFLECTS FROM A PLANE MIRROR?

HOW IS THE ANGLE OF REFRACTION DIFFERENT FROM THE ANGLE OF REFLECTION? The angle of refraction (R) is measured between the refracted ray and the normal.

PLANE MIRRORS (CONTINUED)

CONCAVE MIRROR (OBJECT FAR FROM MIRROR)

CONCAVE MIRROR (OBJECT NEAR TO MIRROR)

CONVEX MIRROR

CONCAVE LENS

CONVEX LENSES – THE IMAGE FORMED DEPENDS ON THE POSITION OF THE OBJECT

PARTS OF THE EYE

HOW LIGHT ENTERS THE EYE 1.Light Rays are refracted by the cornea, a transparent tissue that covers the iris and pupil. It does most of the focussing for the eye. 2.The sclera is an opaque tissue that is the white region surrounding the iris. 3.Refracted rays enter the eye through the pupil. The iris (the coloured circle of muscle surrounding the pupil) changes the size of the pupil. 4.Pupil becomes larger if light is dim and it becomes smaller if light is bright.

HOW LIGHT ENTERS THE EYE (CONTINUED) 5.The light rays pass though the lens. They converge on the retina. 6.The lens has the ability to fine-tune our focus by automatically changing shape. 7.Lens becomes thicker to help focus light rays from objects that are closer to you. 8.Lens becomes thinner to help focus light rays from objects that are farther away. 9.The image formed on the retina is upside down. Cells in the retina change the image into electrical signals. 10.The electrical signals are sent to the brain along the optic nerve. The brain interprets the signals and changes the image to upright. 11.Blind spot is the area where the optic nerve enters the retina. This area has no light-sensing cells.

BLACK AND WHITE & COLOUR VISION Cells come in 2 basic shapes: rod cells (longer cylindrical) and cone cells (rounder) Rod cells can absorb almost any colour of light, but the brain does not use any of the signals from rod cells to determine colour – just shades of light and dark. In low light conditions, this helps us see shapes and movement. Cone cells allow us to detect colours. The human brain can combine and balance the different colour signals that it receives.

NORMAL VISION When light rays from a distant object enter the eye, the rays are nearly parallel. The lens, which is convex, causes the rays to converge at the retina, producing a sharp image. When light rays from a nearby object enter the eye, the rays are diverging. The lens become thicker giving it a greater ability to converge the light rays to form a clear image.

CORRECTING FOCUS PROBLEMS A person who is near-sighted can see objects clearly when they are close to the eye. This condition occurs when light rays converge before they reach the retina. A concave lens is used to diverge the parallel rays slightly so that the image forms further back on the retina. A person who is far-sighted can see objects clearly when they are far from the eye. This condition occurs when light rays do not converge before they reach the retina. A convex lens is used to converge the rays before they reach the cornea.

ASTIGMATISM & BLINDNESS If the shape of cornea is irregular, light rays can focus in more than one place on the retina. Astigmatism can be corrected using eyeglasses or contact lens or with laser surgery to reshape the cornea. Blindness can be any vision impairment that keeps people from doing important life activities. Snow blindness is a painful condition of temporary partial or complete blindness caused by the overexposure to the glare of sunlight. Night blindness is a condition in which it is difficult or impossible to see in dim light. Colour blindness is the ability to see only in shades of grey. Colour vision deficiency is an inability to distinguish certain colours (i.e. red and green)

EXTENDING HUMAN VISION The screen that is receiving the image must be the correct distance from the lens. Too close, the light rays don’t fully converge, blurred image. Too far away, the light rays begin to diverge, blurred image. Adjusting the distance between the screen and the lens to make a clear image is called focussing.

MICROSCOPES A compound light microscope uses two convex lenses with relatively short focal lengths to magnify small, close objects. To magnify means to cause to look larger than the real size. The objective lens is a convex lens that makes an enlarged image inside the microscope. The light rays from that image then pass through another convex lens in the eyepiece. This magnifies the image even more.

TELESCOPES A telescope magnifies distant objects. In a refracting telescope a convex lens gathers light from the object and focusses it into an enlarged image. The light rays from that image pass through a convex eyepiece lens to magnify the image even more. In a reflecting telescope a concave mirror gathers the light from a distant object. A plane mirror reflects the light gathered by the concave mirror toward the side of the telescope tube. The image that forms there is magnified by a convex eyepiece lens.

BINOCULARS Binoculars are two refracting telescopes mounted side by side. The telescopes in binoculars are shortened by placing prisms inside that serve as plane mirrors. Rather than travelling down the long tube of a telescope, the prisms reflect the light in binoculars back and forth in a shorter tube.

CAMERAS A camera works in much the same way as the eye to form an image. A camera has an opening to let light in. The size of the opening can be controlled to let in more or less light. A camera has a convex lens to gather and focus light into an image. A camera has a light detector that acts like the retina. The light detector changes light into electric signals which are changed into a picture.

LASER LIGHT Laser light is light that has just one wavelength. It can carry a large amount of energy making it useful for eye surgery. Laser light is also used in optical fibre technology. An optical fibre is a very thin transparent tube that can transmit laser light from one place to another. Laser light enters one end of the fibre and keeps reflecting off the smooth inside walls until it reaches the other end. This type of reflection is called total internal reflection.