LIGHT

CLASS VII

Q1. WHAT ARE THE CHARACTERISTIC FEATURES

OF LIGHT?

A1. IT IS A FORM OF ENERGY.

*IT SHOWS RECTILINEAR PROPAGATION WHICH

MEANS IT ALWAYS TRAVELS IN STRAIGHT LINE.

*IT SHOWS REFLECTION, REFRACTION AND

DISPERSION.

Q2. EXPLAIN REFLECTION AND ITS TYPES?

A2. THE BOUNCING OFF OF LIGHT FROM A

SURFACE IS CALLED REFLECTION.

*THE PATH OF LIGHT FALLING ON A SURFACE IS

CALLED INCIDENT RAY OR i-ray WHILE THE RAY

OF LIGHT THAT GETS REFLECTED BACK IS

CALLED REFLECTED RAY OR r-ray.

THE TWO TYPES OF REFLECTION ARE-

1. REGULAR REFLECTION

-IT OCCURS ON SMOOTH SURFACES.

-THE REFLECTED RAYS ARE PARALLEL TO EACH

OTHER.

2. IRREGULAR REFLECTION

-IT OCCURS ON ROUGH SURFACES.

-THE REFLECTED RAYS ARE NOT PARALLEL TO

EACH OTHER AND ARE UNPREDICTABLE.

Q3. EXPLAIN THE LAWS OF REFLECTION.

A3.

* THERE ARE TWO LAWS OF REFLECTION:

1. THE ANGLE OF INCIDENCE IS EQUAL TO

ANGLE OF REFLECTION.

2. THE INCIDENT RAY, THE REFLECTED RAY AND

THE NORMAL LIE ON THE SAME PLANE, AT THE

POINT OF INCIDENCE.

Q4. MENTION SOME PROPERTIES OF IMAGE

FORMED BY PLANE MIRROR.

A4. THE PROPERTIES OF IMAGE FORMED BY

PLANE MIRROR ARE-

1. THE IMAGE IS ALWAYS VIRTUAL AND ERECT.

2. THE IMAGE SHOWS LATERAL INVERSION

THAT IS , A SIDEWAYS CHANGE IN WHICH LEFT

APPEARS TO BE RIGHT AND VICE VERSA. 3. THE

IMAGE FORMED IS OF SAME SIZE AS THE

OBJECT IS.

4. THE IMAGE IS AS MUCH BEHIND THE

MIRROR AS THE OBJECT IS IN FRONT OF IT.

Q5. DISTINGUISH BETWEEN REAL AND VIRTUAL

IMAGE.

A5. REAL IMAGE

*IT IS ALWAYS INVERTED.

*IT CAN BE TAKEN/FORMED ON THE SCREEN.

*IT IS FORMED BY THE ACTUAL INTERSECTION

OF THE RAYS.

*IT IS ALWAYS FORMED IN FRONT OF THE

MIRROR.

VIRTUAL IMAGE

*A VIRTUAL IMAGE IS ALWAYS ERECT.

*IT CANNOT BE TAKEN ON THE SCREEN.

*THE RAYS, DON'T INTERSECT BUT THEY JUST

SEEM TO BE MEETING AT A POINT.

*IT IS ALWAYS FORMED BEHIND THE MIRROR.

Q6. WHY IS AN IMAGE FORMED BY A PINHOLE

CAMERA INVERTED?

A6. AS THE LIGHT SHOWS RECTILINEAR

PROPAGATION SO,THE LIGHT RAY COMING

FROM THE OBJECT CROSSES THE PINHOLE AND

CONTINUES TO MOVE IN A STRAIGHT LINE

WITHOUT CHANGING ITS POSITION.

*WHEN THEY FALL ON THE SCREEN, THE RAYS

FROM THE TOP HALF ARE BELOW THE RAYS

FROM THE BOTTOM HALF.

*THUS, THE IMAGE FORMED IS INVERTED.

Q8. SHOW THE IMAGES FORMED BY CONCAVE

MIRROR?

A8. A CONCAVE MIRROR IS A MIRROR WHOSE

REFLECTING SURFACE IS CURVED INWARDS.

HERE,

C=CENTRE OF CURVATURE

F= FOCUS

P= POLE

C = 2F

OR, F = C/2

*THE IMAGES FORMED BY A CONCAVE MIRROR

DEPENDS ON THE DISTANCE OF THE OBJECT

FROM THE MIRROR.

1. WHEN THE OBJECT IS AT INFINITY.

*THE IMAGE IS FORMED AT FOCUS (F).

*IT IS REAL INVERTED AND POINT SIZED.

2. WHEN THE OBJECT IS BETWEEN INFINITY

AND C.

* THE IMAGE FORMED IS REAL AND INVERTED,

*IT IS BETWEEN F AND C.

*DIMINISHED.

3. WHEN THE OBJECT IS AT C.

*THE IMAGE FORMED IS ALSO AT C.

*REAL AND INVERTED AND OF THE SAME SIZE

AS THE OBJECT IS.

4. WHEN THE OBJECT IS BETWEEN F AND C.

*THE IMAGE IS BEYOND C.

* IT IS REAL, INVERTED AND ENLARGED.

5. WHEN THE OBJECT IS AT F.

*THE IMAGE IS FORMED AT INFINITY.

*REAL, INVERTED AND MAGNIFIED..

6. WHEN THE OBJECT IS BETWEEN F AND

P(POLE).

*THE IMAGE FORMED IS VIRTUAL AND ERECT.

*IT IS FORMED BEHIND THE MIRROR AND IS

MAGNIFIED.

Q9. MENTION THE USES OF CONCAVE MIRROR.

A9. 1. THEY ARE USED IN INSTRUMENTS OF

DENTISTS.

2. THEY ARE USED AS SHAVING MIRRORS.

3. THEY ARE ALSO USED IN SOLAR COOKERS.

Q10. WHY DO WE USE CONVEX MIRRORS AS

REAR VIEW MIRRORS IN VEHICLES ?

A10. WE USE A CONVEX MIRROR AS THE REAR

VIEW MIRRORS IN VEHICLES BECAUSE THE

IMAGE FORMED IS VIRTUAL, ERECT AND

DIMINISHED IMAGE.

*THUS,WE HAVE A LARGER VIEW OF AREA OR

TRAFFIC BEHIND.

Q11. WHAT IS REFRACTION ?

A11. 1. THE BENDING OF LIGHT AS IT TRAVELS

FROM ONE TRANSPARENT MEDIUM TO

ANOTHER THAT IS FROM A RARER TO A

DENSER MEDIUM OR VICE VERSA IS CALLED

REFRACTION. IT OCCURS AT THE INTERFACE.

2. WHEN LIGHT TRAVELS FROM RARER TO

DENSER MEDIUM, IT BENDS TOWARDS THE

NORMAL.

3. WHEN LIGHT TRAVELS FROM DENSER TO

RARER MEDIUM, IT MOVES AWAY FROM THE

NORMAL.

Q12. CAN YOU SEE THE IMAGE FORMED IN THE

GIVEN PICTURE. EXPLAIN THE CONCEPT BEHIND

THIS.

A12.THE GIVEN PICTURE SHOWS A SPOON

DIPPED IN A WATER WHICH SEEMS TO BE BENT

.

*IT IS BECAUSE OF REFRACTION.

*AS LIGHT PASSES FROM RARER TO DENSER

MEDIUM THAT IS FROM AIR TO WATER, IT

BENDS TOWARDS THE NORMAL.

*THUS THE IMAGE SEEM TO BE BENT.

Q13. WHAT IS WHITE LIGHT ?

A13. A WHITE LIGHT IS A COMBINATION OF

SEVEN WAVELENGTHS CORRESPONDING TO

SEVEN COLOURS- VIOLET, INDIGO, BLUE,

GREEN, YELLOW, ORANGE AND RED. WE

NORMALLY GET WHITE LIGHT AS A VISIBLE

SPECTRUM FROM THE SUN.

THE SEVEN COLOURS APPEAR TOGETHER TO BE

WHITE BECAUSE ALL THE SEVEN COLOURS

TRAVEL WITH THE SAME VELOCITY IN THE AIR

WHILE THEIR VELOCITIES CHANGE AS THEY

TRAVEL THROUGH DIFFERENT MEDIA.

Q14. WHAT IS SPECTRUM ?

A14. WHEN WHITE LIGHT DISPERSES OR

BREAKS INTO ITS SEVEN COLOURS THAT IS

VIBGYOR ,ON PASSING THROUGH THE DENSER

MEDIUM, A SPECTRUM IS FORMED.

*FOR Eg. RAINBOW.

Q15. WHY DOES WHITE LIGHT SPLIT INTO

SEVEN COLOURS ON PASSING THROUGH

DENSER MEDIUM ?

A15. WHEN WHITE LIGHT TRAVELS THROUGH

AIR OR VACUUM, THE SEVEN WAVELENGTHS

TRAVEL WITH THE SAME VELOCITY BUT WHEN

IT ENTERS GLASS, OIL OR WATER(ANY DENSER

MEDIUM) THE WAVELENGTHS TRAVEL WITH

DIFFERENT VELOCITIES.

*RED IS THE FASTEST AND SO IS THE FIRST TO

EMERGE WHILE VIOLET IS THE SLOWEST AND

SO IS THE LAST TO EMERGE.

*THUS, A SPECTRUM IS FORMED.

LENSES:

THEIR ARE TWO KINDS OF LENSES

1. CONVEX LENS.

2. CONCAVE LENS.

**IMAGE FORMATION BY CONVEX LENS:

1.WHEN OBJECT IS AT INFINITY.

*IMAGE IS FORMED AT F2.

IT IS REAL , INVERTED AND POINT SIZED.

2. WHEN OBJECT IS BEYOND 2F1.

*IMAGE IS FORMED BETWEEN F2 AND 2F2.

IT IS REAL , INVERTED AND DIMINISHED.

3. WHEN OBJECT IS AT 2F1.

*IMAGE IS FORMED AT 2F2.

IT IS REAL ,INVERTED AND OF THE SAME SIZE AS

THE OBJECT IS.

4. WHEN THE OBJECT IS BETWEEN 2F1 AND F1.

*IMAGE IS FORMED BEYOND 2F2.

IT IS REAL , INVERTED AND MAGNIFIED.

5.WHEN THE OBJECT IS AT F1.

*IMAGE IS FORMED AT INFINITY.

IT IS REAL, INVERTED AND HIGHLY MAGNIFIED.

6. WHEN THE OBJECT IS BETWEEN F1 AND O.

*IMAGE IS FORMED BEYOND 2F1.

IT IS VIRTUAL , ERECT AND MAGNIFIED.

Q16.WHICH TYPE OF LENS IS USED IN

MAGNIFYING GLASS.WHAT KIND OF IMAGE IS

FORMED WHEN WE PLACE A MAGNIFYING

GLASS CLOSE TO AN OBJECT?

A16.CONVEX LENS IS USED IN MAGNIFYING

GLASS.

*WHEN WE PLACE A MAGNIFYING GLASS

CLOSE TO AN OBJECT, THE IMAGE FORMED IS

VIRTUAL, ERECT AND MAGNIFIED.

Q17.YOU HAVE A CONVEX LENS.WHERE WILL

YOU PLACE AN OBJECT TO SEE AN ERECT AND

MAGNIFIED IMAGE?

A17. WE WILL PLACE THE OBJECT BETWEEN F1

AND O.

DO IT YOURSELF

1. GIVE AN EXAMPLE THROUGH WHICH YOU

CAN SHOW RECTILINEAR PROPAGATION OF

LIGHT.

2. WHY ARE CONCAVE MIRRORS USED BY

DENTISTS?

3. YOU HAVE A CONCAVE MIRROR.WHERE WILL

YOU PLACE AN OBJECT TO SEE AN ERECT AND

MAGNIFIED IMAGE?

4. COMPLETE THE FOLLOWING DIAGRAM.

**ALL DIAGRAMS MUST BE DRAWN IN COPY

WITH PENCIL, RULER AND COMPASS.ARROWS

ARE MUST.**