3 d) light and sound Flashcards
3.12 understand that light waves are transverse waves which can be reflected and refracted
Light waves are transverse wave that is emitted from luminous or non-luminous objects. Light waves are transverse wave and like all waves, they can be reflected and refracted
3.13 use the law of reflection (the angle of incidence equals the angle of reflection)
http://www.shawonnotes.com/IGCSE_Physics/physics_images/Reflection.png
The law of reflection states that:
1) The incident ray, reflected ray and normal all lie in the same plane.
2) The angle of incidence is equal to the angle of reflection.
3.14 construct ray diagrams to illustrate the formation of a virtual image in a plane mirror
Types of images:
Virtual images: Image through which the rays of light don’t not actually pass is called virtual image. Example: Image formed in the mirror. Virtual images cannot be produced on a screen.
Real images: Images created with rays of light actually passing through them are called real images. Example: cinema screen.
Properties of an image in a plane mirror
- The image is as far behind the mirror as the object is in front
- The image is the same size as the object
- The image is virtual – that is, it cannot be produced on a screen
- The image is laterally inverted – that is, the left side and right side of the image appear to be interchanged.
Constructing ray diagrams
Things we include in ray diagrams of a plain mirror: object, observer’s eye or some indication, plane mirror, image and rays passing object and image.
http://www.shawonnotes.com/IGCSE_Physics/physics_images/construct-ray-diagrams.jpg
3.15 describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms
As a light ray passes from one transparent medium to another, it bends. This bending of light is called refraction. Refraction occurs due to having different speed of light in different medium. For example, light travels slower in glass than in air. When ray of light travels from air to glass, it slows down as it crosses the boundary between two media. The change in speed causes the ray to change direction and therefore refraction occurs. Example:
http://www.shawonnotes.com/IGCSE_Physics/physics_images/AngleRefraction.jpg
The light bends towards the normal as it passes from low-density to high-density(air to glass). The light is refracted and upon emerging from the glass the light bends away from the normal as it passes high density to low-density (glass to air).
Experiment: To demonstrate the refraction of light through a piece of glass block.
Apparatus: Rectangular glass block with one face frosted, two rays boxes, piece of paper, protractor.
http://www.shawonnotes.com/IGCSE_Physics/physics_images/refraction-demonstration.jpg
Procedure:
Place the glass block on a piece of paper with the frosted side down.
Send two narrow rays of light through the glass block as shown in Figure.
Observe the paths of the two rays of light.
Vary the angle of incidence i and measure the angle of refraction r using protractor.
3.16 know and use the relationship between refractive index, angle of incidence and angle of refraction:
n = sin i / sin r
refracive index = sin(incident angle) / sin(refracted angle)
3.17 describe an experiment to determine the refractive index of glass, using a glass block
Experiment: To determine the refractive index of glass, using a glass block.
http://www.shawonnotes.com/IGCSE_Physics/physics_images/refractiveindex-demonstration.jpg
1) Put the glass block on an wooden table which is passed by a white sheet.
2) The border of the block is marked by a pencil.
3) At one border draw a normal and draw three lines to use as incident ray.
4) Set a ray box through anyone of the lines.
5) The ray travels and passes through the glass block and finally emerges from the glass block.
6) The passage of the ray is marked by putting some pins.
7) move the glass block and gain the footprints of the pins to show the passage of the ray.
8) Now using a protractor measure the ∠i and ∠r.
9) Now using, = sin i/sin r ; calculate refractive index.
Ways to improve result:
1) Repeat the experiment, and find the average reading.
2) Plot a graph of sin I against sin r and find the gradient.
3) Vary the value of i and repeat.
3.18 describe the role of total internal reflection in transmitting information along optical fibres and in prisms
Total internal reflection: When light falls on the surface of a lighter medium from denser medium at an angle of incidence greater than critical angle, then the light does not refracts. It rather reflects in the self-medium. This type of reflection is called total internal reflection.
http://www.shawonnotes.com/IGCSE_Physics/physics_images/critical-angle-and-internal-reflection.jpg
Condition of total internal reflection:
Light should fall in the surface of lighter medium from denser medium.
Angle of incidence must be greater than the critical angle.
The prismatic periscope (http://www.shawonnotes.com/IGCSE_Physics/physics_images/prismatic-periscope-new.jpg)
Light passes normally through the surface AB of the first prism (that is, it enters the prism at 90oo. The critical angle for glass is 42o so the ray is totally internally reflected and is turned through 90o. On emerging from the first prism the light travels to a second prism which is positioned such that the ray is again totally internally reflected. The ray emerges parallel to the direction in which it was originally travelling.
The final image created by this type of periscope is likely to be sharper and brighter than that produced by a periscope that uses two mirrors. Because in mirrors, multiple images are formed due to several partial internal reflections at the non-silvered glass surface of the mirror.
OPTICAL FIBRES (http://www.shawonnotes.com/IGCSE_Physics/physics_images/total-internal-reflection-optical-fibre.jpg)
Optical fibre uses the property of total internal reflection. This is very thin strand composed of two different types of glass. The inner core is more optically dense than the outer one. As the fibres are narrow, light entering inner core always strike the boundary of the two glasses at an angle greater than critical angle. This technique is used to send information very fast at the speed of light.
Optical fibres are used in endoscopes and telecommunications.
3.19 explain the meaning of critical angle c
Critical angle is an incident angle at which the incident ray is refracted and the refracted angle is equal to 90 degree in condition that the light falls on the surface of a lighter medium from denser medium.
3.20 know and use the relationship between critical angle and refractive index:
sin c = 1/n
sin (critical angle) = 1/ refractive index
3.21 understand that sound waves are longitudinal waves and how they can be reflected and refracted
Sound waves are longitudinal waves. Like other waves they can also be reflected and refracted. Sound waves reflect when they bounce back from a surface so that the angle of incident is equal to the angle of reflection. A reflected sound wave is called an echo. Sound waves refract when it changes direction while travelling across a high dense medium. Sound waves are diffracted when they spread while travelling through a narrow space such as doorway.
3.22 understand that the frequency range for human hearing is 20 Hz – 20,000 H
An average person can only hear sound that have a frequency higher than 20Hz but lower than 20000 Hz. This spread of frequency is called audible range. Frequency higher than 20000 Hz which cannot be heard by humans are called ultrasounds. Frequency lower than 20 Hz that cannot be heard by humans are called infrasound.
3.23 describe an experiment to measure the speed of sound in air
Experiment: To measure the speed of sound by direct method
Apparatus: Starting pistol, stopwatch, measuring tape.
Procedure:
1) By means of measuring tape, observers are positioned at known distance apart in an open field.
2) First observer fires a starting pistol.
3) Second observer seeing the flash of the starting pistol, starts the stopwatch and then stops it when he hears the sound. The time interval is then recorded.
Ways to improve:
1) Repeat the experiment a few times and compute the values of the speed of sound for each experiment. Find the average value. This procedure minimizes random errors in finding the time interval between seeing the flash and hearing the sound.
2) Observers exchange positions and repeat experiment. This procedure will cancel the effect of wind on the speed of sound in air.
