2. U KNOW WHY OBJECTS FAR
FROM US LOOKS SMALLER
WHILE THE OBJECTS CLOSE
TO US SEEMS LARGER
THE ANSWER TO
THIS QUESTION IS
VISUAL ANGLE,
3. VISUAL ANGLE IS DEFINED AS THE ANGLE
SUBTENDED BY AN OBJECT ON A HUMAN EYE .
4. ■ The apparent size of object due to visual angle is called angular size of
object.
■ This angle is measured in degrees in S.I system
■ The apparent size of an object depends upon its visual angle:
■ ⇢ Greater the distance of object smaller will be visual angle , Smaller the
distance of object greater will be visual angle.
→ larger the visual angle greater will be apparent size of object smaller
the visual angle smaller will be apparent size of object.
■
5. The sun. the moon and all other stars appear smaller to us but, in fact they are very large because they make a smaller visual angle on
our eye.For example, the Moon has an angular size of
approximately 30 arcminutes But it’s actual size is 1737.4km
6. From visual angle it is concluded that the objects appear
clear if they are closer to us but this near distance is
limited to least distance of distinct vision beyond this
distance objects do not appear clear.
What is least distance
of distinct vision?
Closet distance at which
normal vision can
comfortably look at
something
It is equal to 25cm
It increases with increase in age as, For young people,
the least distance of distant vision will be within 25cm
which however it varies with age. For infants, the least
distance of distinct vision is about 5 to 8 cm
7. ANGULAR MAGNIFICATION
■ The ratio of visual angle subtended by the image seen
through a magnifying glass to the visual angle subtended by
the object when placed at the least distance of distinct
vision, when seen through naked eye.
■ M=
𝛽
𝛼
=
𝜃
𝜃"
■ It has no unit.
8. We learned that an object does not appear clear if it
is placed near than the least distance of distinct vision
but, there are some instruments which make an object
clear by bringing more closer than 25cm and a
SIMPLE MICROSCOPE Is one of them.
9. SIMPLE MICROSCOPE
A simple microscope is a magnifying glass
which is used to see details of object by
bringing it closer than 25 cm.
The magnifier is a double convex lens with a
short focal length,
It forms a virtual and erect image as object
is placed with in the focal length of lens
Generally it’s magnification is between 2
and 6
10. MAGNIFICATION OF SIMPLE
MICROSCOPE
■ If α and β are the two angles
subtended by the object when
seen through the lens (magnifying
glass) and when viewed directly,
then angular magnification is
equal to
M=
β
α
When angles are small, then they
are nearly equal to their tangents.
α= tan α =
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡
=
𝑂
𝑑
β =tan β =
𝑠𝑖𝑧𝑒 𝑜𝑓𝑖𝑚𝑎𝑔𝑒
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓𝑖𝑚𝑎𝑔𝑒
=
𝐼
𝑞
11. Since the Image is at the least distance of
distinct vision, hence
q=d
therefore β =
𝐼
𝑑
the angular magnification
M=
𝐼
𝑑
𝑜
𝑑
=
𝐼
𝑂
12. as we already know
𝐼
𝑂
=
𝑞
𝑝
M=
𝑞
𝑝
=
𝑑
𝑝
for virtual images lens formula is written as
1
𝑓
=
1
𝑝
−
1
𝑞
