NEW SIGN CONVENTION OF CONVEX AND CONCAVE LENS.

New sign convention for convex and concave lenses.

1. The optical centre of the lens matches with the origin of two dimensional cartesian system ( the point of intersection of x-axis and y-axis).

2. The object should be placed at the left side of the lens( towards negative x-axis).

3. The distances are measured from the optical centre of the lens( the origin). Assign the signs according to the signs of the segments of the axes.(x or y axis).

For CONVEX LENS.

4. The object distance (u) is always negative.

5. The image distance (v) is positive for real images.

6. The image distance (v) is negative for virtual image.

7. The focal length (f) is positive. (Second local length)

For CONCAVE LENS.

8. The object distance (u) is always negative.

9. The image distance (v) is always negative as it forms virtual image.

10. The focal length (f) negative. (Second local length)

For OBJECT and IMAGE lengths.

11. The length of the image is positive as it is always kept erect.

12. The length of the real image is negative as it is inverted.

13. The length of the virtual image is positive.

For MAGNIFICATION, (m=Image length÷ object length)

14. m is negative for real images.

15. m is positive for virtual image.

For NUMERICALS,

16. Do not assign any sign for unknown quantities.

LENS FORMULA

17. Magnification (m)= Size of the image÷size of the object.

18. m= image distance ÷object distance.

19. Lens formula: 1/f= 1/v-1/u.

UNIFORM CIRCULAR MOTION

In uniform circular motion, 1. Speed is constant. 2. Velocity is variable and acts along tangential direction at a point. 3. Centripetal Force and acceleration act along the radius towards the centre if the circular path. 4. Centrifugal force appears to act radially outwards. It is a virtual ,fictitious force, not a real force.

ECHO IN LIVING ROOM

Why is echo not heard in our living rooms? To hear echo, the minimum distance between the source and the reflector must be 17m. When this distance is more than 17m, the reflected sound can be distinguished from the original. As the length of our rooms is less than 17m, we don't hear echo.

CONDITIONS FOR TOTAL INTERNAL REFLECTION. CLASS 10

1. The light ray should travel from denser to rarer medium. 2. The angle of incidence in the denser medium should be greater than the critical angle for the given pair of media.

PHYSICS

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