class viii science

1. Light
Dhatri bhat

2. learning objectives
what makes things visible?
Laws of reflection
Regular and diffused reflection
Reflected loght can be reflected again
Multiple images
Sunlight- white or coloured
What is inside our eyes
Care of the eyes

3. learning objectives
Visually impaired persons can read and write
Braille system
Assignment

4. By dhatri
For things to be visible there are two
factors. First is when an object
reacts with its surroundings. An
example of this would you cannot
see a black colour cat at night but it
becomes visible when the
background is white.
The second factor is visible light.
There is a set of a band of lights that
are detected by the human eye.
When these lights are detected by
the eye, it gets converted into
wavelengths that are understood by
our brain.
What makes
things visible?

5. 1. The law of reflection defines
that upon reflection from a
smooth surface, the angle of the
reflected ray is equal to the
angle of the incident ray, with
respect to the normal to the
surface that is to a line
perpendicular to the surface at
the point of contact.
2. The reflected ray is always in the
plane defined by the incident ray
and the normal to the surface at
the point of contact of the
incident ray.
Laws of reflection

6. ● Regular Reflection:
● The plane mirrors with a smooth surface
produce this type of reflection. In this
case, the image is clear and is very
much visible. The images produced by
plane mirrors are always virtual, that is
they cannot be collected on a screen.
● In the case of curved mirrors with a
smooth surface, we can see the images
of reflection either virtually or really.
That is, the images produced by curved
mirrors can be either real (collected on a
screen and seen), or virtual (cannot be
collected on a screen, but only seen).
Continued

7. ● Irregular Reflection:
● Unlike mirrors, most natural
surfaces are rough on the scale
of the wavelength of light, and,
as a consequence, parallel
incident light rays are reflected
in many different directions
irregularly, or diffusely. Hence,
diffuse reflection helps in seeing
the objects and is responsible
for the ability to see most
illuminated surfaces from any
position.
Regular and irregular
reflection

9. ● What is Angle of Reflection?
● The angle of reflection Θr of a
ray is the angle measured from
the reflected ray to the normal
surface.
● Calculation of Angle of
Incidence and Angle of
Reflection
● The angle of incidence and the
angle of reflection are
calculated by drawing a normal
line that is perpendicular to the
reflecting surface.
Angle of reflection and
angle of incidence

10. S.No. Regular Reflection Irregular Reflection
1
It occurs when all the reflected
rays from a given smooth
surface are parallel for parallel
incident rays.
It occurs when for a given set of
incident parallel rays, the
reflected rays do not remain
parallel to each other.
2
This occurs from smooth
surfaces like a mirror, silver
spoon, etc.
This occurs from rough surfaces
like wood, table, door, book, etc.
3 The image is formed and seen.
It helps to see objects. No
images.

11. Difference
between concave
and convex
mirrors

12. Concave Mirrors:
Concave mirrors give real, inverted images if the
object is beyond the focus and a virtual, erect,
enlarged image if the object has a distance less
than the focal length from the pole of the mirror.
Uses of Concave Mirrors:
1.Concave mirrors are used in torches,
searchlights, and headlights of vehicles to get
powerful parallel beams of light.
2.Concave mirrors are also used as shaving
mirrors to see a larger image of the face.
3.Dentists use concave mirrors to see bigger
images of the teeth of the patients.
4.4) Large concave mirrors are used to focus
sunlight to produce heat in the solar furnaces.
Convex Mirrors:
Convex Mirrors always give a virtual, erect,
diminished image of the object behind the
mirror.
Uses of Convex Mirrors:
1.The convex mirror is used as a side-view
mirror in vehicles to give a smaller view of the
vehicles coming from behind.
2.They are used in shops and supermarkets
and any other place where there is a
requirement for detecting burglars.
3.Convex mirrors are used in making lenses of
sunglasses.
4.Convex mirrors are used in magnifying
glasses, and telescopes.
5.Convex mirrors are used to reflect street
light; because they can reflect over a wide
area.
6.Convex mirrors are kept at the street corners
to avoid collisions.

13. Uses of reflection

14. Uses of Reflection:
1.Reflection is used in periscopes to view
advancing enemies on the battlefield from a
safe position.
2.Reflection is the reason why we see objects.
3.Reflection by a concave mirror and a convex
mirror has many uses as listed above.
4.Reflection helps in medical diagnosis and
optical communications.
5.Light and Sound both follow the law of
reflection, both being waves.
6.Using the law of reflection for sound and
light, we can measure the distances accurately
to objects.
7.Reflection is the reason why we hear the
echo of sound.
When light passes from denser medium to
lighter medium at an angle more than the
critical angle required for refraction, then the
light is reflected back into the denser medium.
This is a phenomenon called Total Internal
ReflectionThe phenomenon, total internal
reflection, is taken advantage in piping light in a
curved path. The light directed down a narrow
fiber of glass or plastic repeatedly reflects from
the fiber-air interface at larger than the critical
angle. Optical fibers can transmit light over long
distances without any loss of intensity.
Sequences of light pulses are used to transmit
information through an optical fiber network
with the help of this total internal reflection.
Medical instruments like ‘endoscopes’ use the
total internal reflection of light through an
optical fiber bundle to image internal organs.

15. Regular and
diffused
reflection

16. When all the parallel rays reflected from a
plane surface are not parallel, the reflection is
known as diffused or irregular reflection.
Reflection from a smooth surface like that of a
mirror is called regular reflection.
Regular reflection
•In regular reflection, the reflected rays are
parallel to each other.
•Regular reflection occurs from a nicely
polished regular reflective surface.
Diffused reflection
•In diffused reflection, the reflected rays are
not parallel to each other.
•It occurs from a diffused or irregular surface.

17. Sunlight – white
coloured

18. When the sun rays enter the earth, they get
distorted by the earth’s atmosphere, air
molecules, dust, smoke and pollution. We know
that different colors of the spectrum have
different wavelengths. The short-wavelength
blue and violet are scattered more than colors
at the lower end of the spectrum which are less
easily scattered.
•Noon
•: If the sun is high overhead in the sky, the rays
suffer the least amount of interference
because the distance covered by them is the
least at that point in time. Consequently, the
blue light is scattered and the sky appears blue
whereas the color of sunlight appears yellow.
•Morning and Evening
•: During the sunrise and sunset, the sun is near
the horizon due to which the light rays have to
travel a longer distance through the
atmosphere. As a result, there is more
interference which causes more scattering.
Consequently, most of the colors, including the
blue as well as the yellow are scattered leaving
the red light. Hence, the color of sunlight
appears in the tinges of orange and red during
dawn and dusk.

19. What is inside
our eyes?

20. The eye is an important and one of the most
complex sense organ that we humans are
endowed with. It helps us in visualizing
objects and also helps us in light perception,
color, and depth perception. Besides, these
sense organs are pretty much similar to
cameras, and they help us see objects when
light coming from outside enters into them.
That being said, it is quite interesting to
understand the structure and working of a
human eye. It helps us also in understanding
how a camera also actually functions. Let’s
have a glance on the human eye – it’s
structure and function.

21. •clera: It is the outer covering, a protective
tough white layer called the sclera (white part
of the eye).
•Cornea: The front transparent part of the
sclera is called cornea. Light enters the eye
through the cornea.
•Iris: A dark muscular tissue and ring-like
structure behind the cornea are known as the
iris. The colour of the iris actually indicates the
colour of the eye. The iris also helps regulate
or adjust exposure by adjusting the iris.
•Pupil: A small opening in the iris is known as
a pupil. Its size is controlled by the help of iris.
It controls the amount of light that enters the
eye.
•Lens: Behind the pupil, there is a transparent
structure called a lens. By the action of ciliary
muscles, it changes its shape to focus light on
the retina. It becomes thinner to focus distant
objects and becomes thicker to focus nearby
objects.
•.
•Retina: It is a light-sensitive layer that consists
of numerous nerve cells. It converts images
formed by the lens into electrical impulses.
These electrical impulses are then transmitted
to the brain through optic nerves.
•Optic nerves: Optic nerves are of two types.
These include cones and rods
1.Cones: Cones are the nerve cells that are
more sensitive to bright light. They help in
detailed central and colour vision.
2.Rods: Rods are the optic nerve cells that are
more sensitive to dim lights. They help in
peripheral vision.
At the junction of the optic nerve and retina,
there are no sensory nerve cells. So no vision is
possible at that point and is known as a blind
spot.

22. An eye also consists of six muscles. It includes
the medial rectus, lateral rectus, superior
rectus, inferior rectus, inferior oblique, and
superior oblique. The basic function of these
muscles is to provide different tensions and
torques that further control the movement of
the eye.
Function of the Human Eye
As we mentioned earlier that the eye of a
human being is like a camera. Much like the
electronic device, the human eye also focuses
and lets in light to produce images. So
basically, light rays that are deflected from or
by distant objects land on the retina after they
pass through various mediums like the cornea,
crystalline lens, aqueous humor, the lens, and
vitreous humor.
Having different refractive indexes is what
bends the rays to form an image. The light rays
finally are received and focused on the retina.
The retina contains photoreceptor cells called
rods and cones and these basically detect the
intensity and the frequency of the light. Further,
the image that is formed is processed by
millions of these cells, and they also relay the
signal or nerve impulses to the brain via the
optic nerve. The image formed is usually
inverted but the brain corrects this
phenomenon. This process is also similar to
that of a convex lens.
In any case, now that we have learned
something about the human eye, each eye is
very important, and they play a distinct part in
helps humans to see.

23. Care of the eyes

24. How to take proper care of eyes?
1.Read at Normal Distance.
2.Never bring book too close or too far.
3.Do not rub your eyes. If dust goes in eyes,do
not rub it.Instead clean it with water.
4.Do not look at bright light. We should not look at
bright light directly like the sun.
5.Do not read in dull light.
6.Eat food Rich in Vitamin A.

25. Multiple images

26. Light reflected off one mirror can easily be
reflected again off another, and another,
and another... This creates many images
of a single object.
In this activity, students learn about the
relationship between mirror angles and
the number of images produced

27. Visually imaried
persons can read
and write

28. Braille is a tactile reading and writing system. It
opens up the written word to blind and visually
impaired people.
Here are ten facts about braille to help you
learn more about this 200-year-old code which
is still a key to literacy and independence for
thousands of blind and visually impaired people
around the world today.
1. Braille was developed by Louis Braille in
the 1820s when he was a pupil at the Royal
Institute for Blind Youth in Paris. Before he
developed his revolutionary code, various
systems mostly using raised print letters were
used to help blind people read. For example,
Valentin Haüy’s tactile books featured
embossed versions of the Roman alphabet.
Louis Braille’s code was designed for tactile
recognition – not visual recognition – and finally
enabled blind and visually impaired people to
also write independently.
2. Braille is not a language. It is a tactile code
enabling blind and visually impaired people to read
and write by touch, with various combinations of
raised dots representing the alphabet, words,
punctuation and numbers. There are braille codes
for the vast majority of languages – some symbols
have different meanings for aspects such as
accented letters, depending on the language. In its
simplest form one letter is represented by one
symbol, however contracted braille provides some
shortening.
3. It’s recommended to learn braille by touch if
you’re losing your vision but still have some
sight remaining when you start. As a tactile code,
it will only be of use if you can read and write it via
touch if sight is lost completely or reading large print
is no longer possible.

30. What is braille
system?

31. Braille is a tactile writing system used by
people who are visually impaired. It is
traditionally written with embossed paper.
Braille users can read computer screens
and other electronic supports using
refreshable braille displays