CRITICAL ANGLE FOR DIFFERENT COLOURS OF LIGHT

 CRITICAL ANGLE:

The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.

FACTORS:

The critical angle of a medium depends on

1. The relative refractive index of the denser medium with respect to the rarer medium. Greater the refractive index, smaller the critical angle. 

For example: refractive index of glass is 1.5, critical angle is 42° and for diamond, refractive index is 2.4, the critical angle is 24°

2. The colour(wavelength) of the light. Longer the wavelength, greater  is the the value of the critical angle.

For example:  Red has longest wavelength among seven colours, so it has maximum value of the critical angle for a medium.

Atmospheric pressure

 What is atmospheric pressure?

Answer: The force exerted per unit area on the surface of the earth due to the weight of air  column above it is called atmoshpheric pressure.

Its SI unit is N/m² or pascal. Other units are dyne/cm², mm of mercury, bar, torr,

Unstable Hydrogen isotope can not emit alpha rays. Why?

 QUESTION: Unstable hydrogen can not emit alpha particles Why ?

Unstable hydrogen, though radioactive, can not emit alpha rays because during alpha decay two protons and two neutrons are emitted which are not present in the hydrogen atom. Hydrogen  has only one proton in its nucleus.

EARTHEN POT: An earthen pot keeps water cool..

 QUESTION: An earthen pot keeps water cool. How ?

Answer: The earthen pot has fine pores through which water comes out to the outer surface where evaporation takes place. During evaporation, the liquid absorbs heat from the surrounding keeping it cool.

g and G, The differences. Class 9

 

The acceleration due to gravity (g) 

1. It is the acceleration caused by the gravitational force.

2. Its value depends on the mass and radius of a planet or heavenly body.

3. Its value changes from place to place.

4.Its a vector quantity.

The universal gravitational constant(G)

1. It is numerically equal to the force of attraction between two bodies each of mass 1kg separated by distace of 1m.

2. Its value does not depend on the mass and size of bodies. 

3. Its value is constant everywhere in the universe. 

4. It's a scalar constan.

3. 

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.

DOES CONVEX LENS FORM VIRTUAL IMAGES?

Question: Does convex lens form virtual images?

Answers: Yes, convex lens can form virtual image when the object is placed at the position between the first focus and the optical centre of the lens. The image is upright and magnified. Its formed on the same side of the lens where object is placed.

REVISION QUESTIONS ON KINETIC THEORY OF MATTER CLASS 8

 PHYSICS, CLASS 8

ALL QUESTIONS ARE COMPULSORY.

Q1. WHY DO WE SMELL HOT FOOD FROM A DISTANCE? 2

Q2. WHY IS THERE NO CHANGE IN TEMPERATURE DURING THE MELTING  PROCESS? 2

Q3. DIFFERENTIATE BETWEEN BOILING AND EVAPORATION WITH 3POINTS. 3

Q4. STATE AND EXPLAIN ANY THREE POSTULATES OF KINETIC THEORY. 3

Q5. DEFINE TWO TYPES OF INTERMOLECULAR FORCE WITH ONE EXAMPLE OF EACH. 2

Q6 EXPLAIN THE PROCESS OF MELTING ON THE BASIS OF KINETIC THEORY OF MATTER. 2

Q7. EVAPORATION CAUSES COOLING . GIVE REASON WITH ONE EXAMPLE. 2

Q8. WHY DOES A CAMPHOR GET CONVERTED TO FUMES ON HEATING? 2

Q9. WATER ______________HEAT DURING FREEZING AND _________ HEAT DURING BOILING. FILL IN THE BLANK WITH “ GAINS OR LOSES" 2

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Answers: 

A1. The molecules of food in gaseous form are free to move in  large intermolecular space available between them and the air and these molecules show zigzag motion  to different parts of a house. So, we can smell hot food from a distance.

A2. During melting process, the heat given to a solid is utilized to do work against intermolecular force of attraction. The heat energy changes completely into potential energy. Hence temperature does not change.

A3.                   Boiling                                                        I                   Evaporation

a) Boiling is a fast process.                                    I        a) Evaporation is a slow process.

b) Boiling does not  cause cooling                        l       b) Evaporation causes cooling.

c) Boiling takes place at a particular                    I      c) Evaporation takes place at any temperature. 

Temperature 

A4. I) Matter is made of atom or molecule..

Matter is made of small particles called atoms or molecules which cant be seen by naked eyes

ii) The molecules are always inconstant, random motion.

The particles continuously move in a zigzag path .

iii) The molecules possess kintic and potential energy.

Due to their motion the molecules possess kinetic energy. As they interact with other molecules, they change their position and possess potential energy.

A5. I) Force of cohesion : The force of attraction that exists between similar kinds of molecules.  Example: The force of attraction between molecules of water. 

ii) Force of adhesion: The force of attraction that exists between molecules of different substances. Example: The wetting of glass due to the force of adhesion between the molecules of water and glass. 

A6.  When a solid is heated,  the molecules gain kinetic energy and start overcoming the strong intermolecular force of attraction. At a particular temperature, the molecules completely overcome the force of attraction and the molecules becomes free to flow and the solid changes  into liquid . This process is called melting. 

A7. Evaporation causes cooling because the liquid absorbs heat from the body and the surrounding to evaporate. Earthen pot keeps water cool by evaporation of water from the surface of the pot.

A8. Camphor molecules have very weak intermolecular force of attraction. By absorbing  heat from surrounding these molecules overcome the force easily and molecules becomes free like gaseous molecules. Hence it is directly converted to fumes.

A9. Loses, gains.

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REVISION QUESTIONS ON MEASUREMENT CLASS 7

CLASS 7 PHYSICS 

FULL MARKS :20

All questions are compulsory.

Q1. Write the full form of FPS, CGS and MKS system of unit. 3

Q2. State the SI unit of i)  Luminous intensity ii) Temperature iii) Electric current . 3

Q3 A block of mass 1000 kg has volume 2 metre cube. Calculate its density in i) kg/ metre cube   ii) g/cm cube. 3

Q4. A bus covers 200km in 5 hr and a car covers 105 km in 3hr. Which of the two travels faster? 3

Q5. Define density and state its SI unit. 2

Q6. Give reason : The density of iron is more than that of cotton. 2

Q7.  Name two devices to measure the volume of an object. 2

Q8. The space occupied by a solid is called ___________ and the amount of surface of a body is called ______________.  2

Answers:

A1. FPS: Foot, Pound,  Second .  CGS: Centimetre, gram, second.  MKS: Metre, kilogram,  Second.

A2. I) candela   ii) kelvin   iii) ampere

A3.  m =1000kg,  V=2 metre cube.  Density =m/V=1000/2= 500kg /m cube.  Density= 500/1000 g/cm cube.=0.5 g/cm cube.

A4. For the bus, distance= 200km, time= 5hr. Speed of the car=distance/time=200/5=40km/hr. 

       For the car, distance=105km, time= 3hr.  Speed of the car=distance/time= 105/3=35 km/h 

       Hence, the bus is faster.

A5. Density is defined as mass per unit volume of a body. Its SI unit is kg/m cube.

A6. The density of iron is more than that of cotton because the molecules are more closely packed in  iron than in cotton. 

A7. Measuring cylinder and measuring beaker.

A8. Volume, area.

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REVISION QUESTIONS , CHAPTER 1, CLASS 9, MEASUREMENT

CLASS 9, PHYSICS

FULL MARKS : 20

All Questions are compulsory. 

Q1. Define : a) measurement,  b) unit of a physical quantity.  2

Q2. State the SI unit of a) Luminous intensity,  b) solid angle. 2

Q3. Fill in the blanks a) 1 metre= _____________ micrometre.

                                      b) 1 Kg= _____________ metric tonne. 2

Q4. Compare the time period of two pendulum of length 2 m and 18 m. ( g=9.8 metre per square second) 2

Q5. State how does the time period of a simple pendulum depends on a) acceleration due to gravity, 

        b) length of pendulum and   c) amplitude of oscillation. 3

Q6. Define a) amplitude of a simple pendulum, b) frequency  of a simple pendulum  and c) seconds pendulum. 3

Q7. What are the three requirements  for selecting a good unit of a physical quantity. 3

Q8 Write the derived unit of FORCE ( in terms of symbols of basic units) in a) MKS b) CGS and c) FPS system of unit. 3

ANSWERS:

A1. a) Measurement is the process of comparing the value of a physical quantity with a chosen standard value of same kind. B) The standard quantity selected to measure a physical quantity is called  a unit.

A2a) candelabra   b) steradian

A3 a) 10^6 ( ten to the power of six) b) 10^-3

A4. T1/T2=\ /~~l1/l2 =\/~2/18=\/~~1/9=1/3= 1:3

A5. Time period is inversely proportional to the square root of acceleration due to gravity. 

Time period is directly proportional to the square root of the length of a pendulum.

Time period does not depend on the amplitude.

Answers from 6 to 8 for your practise (Please do it yourself ) If you want any answers please do write in comment section.

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CRITICAL ANGLE

The angle of incidence of a ray in a denser medium for which the angle of refraction in rare medium is 90°, is called Critical angle for those pair of media. 

The critical angle depends on 

i) the density of medium.

ii) the colour of the light.

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. 

Total internal reflection (ICSE CLASS 10)

When a ray of light travels from denser to rarer medium at the angle of incidence greater than critical angle for the pair of media, the ray gets reflected back to the denser medium without any loss of energy. This phenomenon is called total INTERNAL reflection.