Notes for Unit 3 : Waves
Notes can be downloaded from the link given below:
Notes Excerpt :
Unit 3: Waves
Study about sound and light
waves, both waves consist of kinetic and energy and transfer energy.
Waves in solid and
liquid:
Wave in liquid in terms of
energy transfer and information. There are two waves which will be studied in detail.
Those are Transverse and longitudinal wave.
Transverse waves:
Light (electromagnetic Spectrum) and water waves are two examples of Transverse waves. Movement of particles is perpendicular to the direction of propagation of a wave.
Longitudinal waves:
Vibration of particles is
parallel to the propagation of waves. Longitudinal waves have areas pf
compression and rarefaction.
Compression: Are areas of high pressure as particles are closer together.
Rarefaction: are areas of low pressure as particles are far apart. Example of longitudinal waves include: sound waves.
Description of Waves:
Amplitude (A): the distance
from peak t the middle or from trough to the middle.
Wavelength (λ):
the distance between two peaks or two troughs.
Frequency (f): is number of waves per second and measured in
hertz (hz).
Time Period (T): time taken to produce one wave denoted by this symbol: λ, pronounced as lambda.
Time period and frequency are related with equation:
Wave speed:
Speed at which energy is transferred or waves moves through the medium. Speed calculated using the equation below:
It can also be calculated using v=d/t
V=velocity
D=distance
T=time taken
Wavefront: is a line or plane
on which the vibrations of every point on it are in phase and in same distance
from the source of wave.
USE OF RIPPLE TANKS
TO SHOW THE RIPPLE IN WATER TANKS:
A motor is placed at the surface of water
to produce waves continuously by vibrating a dipper up and down. A beam helps
in producing straight waves. A light shining at the top of the glass tank helps
observe the pattern of crests and troughs on screen below the tank.
Properties of waves: reflection and refraction.
Reflection:
Reflection is described using
angle of incidence (i) that is the angle between the 90o (normal) at
the surface from where it is reflected (metal plate).
Angle of reflection (r) is
the angle present between normal line and reflected ray.
Angle of incidence (i)=Angle of reflection (r) always.
Refraction:
This happens when density
through which waves passes changes i.e., from deep to shallow water or shallow
to deep water. In both the cases, frequency of waves remains same. The waves
slows down and changes angle. This is called as refraction.
Image below shows: Refraction when waves move from deep to shallow; shorter wavelength, slow-moving in shallow water.
Electromagnetic waves:
The electromagnetic spectrum
A range of frequencies of electromagnetic waves which range from 1 hertz (10-12m for gamma rays) to 10 hertz or more (radio waves-2 kms), consisting of various frequencies and wavelength is known as electromagnetic spectrum.
magnetic and Electric
fields
Sound waves travel through air.
Water ripples along the surface of waves can travel through air. These
electromagnetic waves can travel through vacuum. This energy is carried to us
by changing electric and magnetic fields, which are at right angles to the
direction in which wave is travelling. These are transverse wave. These waves
have similar properties to the usual waves i.e., frequency and wavelength.
However, the speed of light is 3 × 108 m/s. The
other waves are much slower. Light travels slower in glass.
Radio waves:
These waves help transmit
local and national radio broadcast. They have wavelength of few cms to transmit
tv signals and international calls. The shorter-wavelength than radio waves but
larger than infra-red are known as microwaves.
These microwaves are used for mobile internet connection and satellite. They
are also used for cooking. It helps in penetrating further than the infra-red and
so it is more conventional heating-method for cooking. Microwaves are made of
metal to absorb the waves as certain wavelengths are a hazard and could burn by
internally heating up the body tissues.
Infrared waves:
have wavelengths between about 10−4 m and 10−6 m. Anything
warmer than the surrounding object (1000 oC) will liberate heat in the
form of infra-red waves. Prolonged sunlight exposure causes burn due to
infra-red. These sunburns can be avoided by keeping out the sun and wearing
protective clothing.
It is also used in
photography where objects can be detected by the heat liberated especially at
night.
Light waves: visible
light which is used by eyes for vision. This is made up of 7 rainbow colours
(VIBGYOR) Violet light has wavelength of 4 × 10-7 m and red light has wavelength of 7 × 10-7 .These waves are most commonly used for: transmitting information
along optical waves fibres and taking photographs. But very bright light can be
damaging to the eyes so sunglasses help protect eyes from damage.
Ultraviolet waves: wavelengths
shorter than visible light (10−8 m/10−9 m). It is emitted from
objects hotter than 4000 o c.Sources of these radiation
include sun and stars. These rays can also damage eyes causing snow-blindness
(sunburn of retina). Prolonged skin exposure can cause skin cancer. Hence,
wearing protective clothing and sunscreen to block off rays of sun. UV lamps
have fluorescence which help in detecting crime.
X-rays: wavelength of 10-10 m, can cause damage to body
tissues as these penetrate the body easily and helps in taking x-rays these
rays are absorbed by bone helps doctors to detect disorders of body tissues.
Gamma rays: short wavelength emitted from nuclei of atoms. These rays are very
penetrating and are emitted from nuclei of various atoms. These rays can cause
mutation to cells and lead to cancer. These rays have many uses and is widely
used so the amount of exposure needs to be monitored. A radiation badge
monitors the exposure and people involved work behind protective screens.
Experiment to detect the colours of light:
By William Herschel was done
to investigate temperature produced by each colour of visible light. The colour
of light split up when passed through a prism into colours of rainbow. Then a
blackened end of thermometer is kept Infront of each colour and the temperature
increases. Temperature increase is the most towards the end of the red-side of
the spectrum. More temperature increased furthermore which indicated infra-red
rays.
Johann Ritter: other end of
spectrum in which he investigated the effect of light on silver chloride which
blackened by sunlight and furthermore it blackened more quickly when he moved
towards violet-end indicating ultraviolet rays at this end of spectrum.
Reflection of light:
It can be reflected the same way as water as both are transverse waves.
Mirrors can be used to reflect light easily. A ray box is used to produce a
light beam. Ray of light before it strikes the mirror is known as the incident
ray, The angle of incidence can be defined as the angle between the
incident ray and normal. The normal is the line at right angles to the
surface of the mirror. After reflection, the ray is called as reflected ray.
The angle between the normal and reflected ray is called angle of reflection.
Angle of incidence=angle of
reflection and both rays’ line on the
same plane. Shiny and smooth surface help reflect the ray and produce clear images.
Rough surface does not produce a clear image.
Refraction of light:
When light travels through a
medium such as glass the light rays get refracted- it changes it direction.
Light travels faster in air than through a medium. The angle of refraction
depends upon two things:
·
Material.
·
Angle of incidence of ray.
Refraction by a
rectangular block:
Angle I =angle between normal
and incident ray (AB)
Angle r= angle between normal and refracted ray (BC)
METHOD:
a) Set up a ray box and produce a narrow ray.
b) Position he block and mark the rays AB and CD by putting a series
of pencil marks.
c)
Trace around the block and
remove the block.
d) Use a ruler to join the rays AB and CD in such a manner that they
are parallel.
e) Use protractor to mark the two normal and measure the angle of
incidence and angle of refraction.
f)
Record these angles and
repeat with different incident angles.
This experiment can be done
with different shapes of glass block such as semi-circular and triangular prism.
The results would show that:
The light ray is bent towards the normal when light enters and when light leaves the block, it is away from the normal. This occurs as the speed of waves changes.
Refractive index:
Can be calculated for 3 materials: water, glass and diamond. The refractive index (n) is the ratio of speed of light in air to the speed of light in glass. It has no unit. Calculated using:
The previous practical can be used again and tabulated for Sin I/ Sin r.
Draw a graph a graph sin I
(y-axis) and Sin r (x-axis)
Gradient helps compare the
refractive index of glass with other materials.
Refraction by prisms:
Is same as through the glass block the baseline to remember is that:
The
light ray is bent towards the normal when light enters and when light leaves
the block, it is away from the normal. This occurs as the speed of waves
changes.
Total internal
reflection:
Happens when angle of
incidence is too large, all the light rays are reflected back in glass. This is
total internal reflection. If angle of incident if too small, it is bent away
from he normal and if its too big it is reflected within glass.
Critical angle:
If the angle of incidence is
larger than critical angle, it is reflected back into glass (red dotted ray)
but if the incident ray strikes at the critical angle the ray is refracted
along the surface of the block (Blue ray).
Comments
Post a Comment
If you have any doubts, please do let me know.