Education

CBSE-Light Angle of Prism A triangular glass prism consists of three rectangular lateral surfaces and two triangular bases. Lateral faces are inclined to each other and the angle between two lateral surfaces is called the angle of prism.  

CBSE-Light Presbyopia and Astigmatism. Presbyopia is part of the natural aging process of the eye, and can be easily corrected. Technically, presbyopia is the loss of the eye's ability to change its focus to see objects that are near. It is not a disease. It's as natural as wrinkles, and it affects everybody at some point in life. Presbyopia generally starts to appear around age 40. Astimagtism: The cornea is the transparent layer over the colored part of the eye. It bends (refracts) light rays and helps focus the light onto the retina in the back of the eye so people can see. When the cornea is oblong shaped, it causes light rays to focus on two different points on the retina, instead of just one.  As a result, people with significant astigmatism may have distorted or blurry vision.

CBSE-Light Coaxial lenses and its uses Coaxial lenses are lenses having common principal axes. A compound microscope, in its simple form, consists of coaxial lens (two lenses having common principal axes) mounted on the outer ends of two hollow metallic tubes. One of the two lenses acts as the objective and the other as the eyepiece.  

CBSE-Light Common defects of vision that can be corrected by the use of suitable eyeglasses or spectacles There are mainly four common defects of vision that can be corrected by the use of suitable eye glasses or spectacles. These are (i) Myopia or near-sightedness, (ii) Hypermetropia or far-sightedness, (iii) Presbyopia, and (iv) Astigmatism.

CBSE-Light Focussing of an eye the objects lying at various distances An eye can focus the images of the distant objects as well as that of the nearby objects on its retina by changing the focal length of its lens. The focal length of the eye-lens is changed by the action of the ciliary muscles. The ciliary muscles can change the thickness of the eye-lens and hence its focal length changes the converging power of the eye-lens.

CBSE-Light We can see colours because The retina of our eyes has a large number of light sensitive cells. The cells on the retina are of two shapes: (i) Rod shaped and (ii) Cone shaped The rod shaped cells of our retina respond to the intensity of light. The cone shaped cells of our retina respond to colours. These cells make us see colours and to distinguish between them. Thus cones make colour perception possible.

CBSE-Light Phenomenon of vision is made use of in cinematography Persistence of vision is made use of in cinematography. It is the ability of an eye to continue to see the image of an object for a very short duration even after the removal of the object. We are able to see the movie pictures in a cinema due to the persistence of vision. The still pictures taken by a movie camera are projected on the screen at a rate of about 24 pictures per second. Under these conditions, the image of one picture persists on the retina till the image of next picture falls on the screen and so on. Due to this, the slightly different images of the successive pictures present on the film merge smoothly into one another and give us the feeling of moving images. Thus the principle of persistence of vision is made use of, in the motion picture projection in cinematography.  

CBSE-Light Adjusting eye itself to deal with light of varying intensity The amount of light entering the eye is controlled by the pupil. If the intensity of the outside light is low, then the pupil expands to allow more light to enter the eye. If the outside intensity is high, then the pupil contracts so that less light enters the eye.

CBSE-Light Working of a human eye Solution:The light rays coming from the object kept in front of us enter the pupil of the eye and fall on the eye-lens. The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. The light-sensitive cells present in the retina gets activated and generates electrical signals. The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision. The mind interprets the inverted image formed on the retina as an erect object.

 CBSE-Light Accommodation of the eye The ability of the eye to focus on near and distant objects, by adjusting its focal length, is called the accommodation of the eye.

CBSE-Light  Uses of convex mirrors- Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

CBSE-Light   Diffraction of light. The bending of light at the edges of the obstacle (opaque objects).

CBSE-Light Increase the magnification and sharpness of the image in optical instruments Many optical instruments consist of a number of lenses. They are combined to increase the magnification and sharpness of the image. The net power (P) of the lenses placed in contact is given by the algebraic sum of the individual powers P1, P2, P3,... as P = P1+P2+P3.

CBSE-Light    Iris The coloured diaphragm between the cornea and lens.

CBSE-Light   Eye lens It is a transparent lens made of jelly like material.

CBSE-Light  Blind spot The point at which the optic nerve leaves the eye. An image formed at this point is not sent to the brain

CBSE-Light What is pupil? The small hole in the iris is the pupil.

 CBSE-Light Refractive index in terms of velocity and mass density. Refractive index of any medium is the amount of density of the medium which resists the incident light and the ratio of change in velocity when it passes from rarer to denser medium.

CBSE notes-Light Relation between Speed of light in a medium and refractive index.  Speed of the light is inversely proportional to the refractive index of the medium.

 CBSE notes-Light What is the nature, size and position of the image formed of an object at infinity? Explain how it is used as a doctor’s mirror. When an object is placed at infinity from a concave mirror, the image formed is (i) real and inverted (ii) highly diminished and (iii) at the focus When the object is at infinity, the image is formed at the focus of the concave mirror, hence the mirror can concentrate all the parallel rays of the light to its focus. This explains the use of concave mirror as a doctor’s head-mirror. A concave mirror is used as a head-mirror by the doctors to concentrate light on the body parts to be examined.

CBSE notes-Light Principal focus of concave and convex mirror. :Principal focus of a concave mirror (i) The principal focus of a concave mirror is a point on its principal axis to which all the light rays, which are parallel and close to the axis, converge after reflection from the concave mirror. (ii) A concave mirror has real focus. Principal focus of a convex mirror (i) The principal focus of a convex mirror is a point on its principal axis from which a beam of light rays, initially parallel to the axis, appear to diverge after being reflected from the convex mirror.

 CBSE notes-Light What happens when a ray of light falls normally on the surface of a mirror? If a ray of light is incident normally on a mirror, the angle of incident is zero. Hence the angle of reflection is also zero. This means that a ray of light, which is incident normally on a mirror, is reflected back along the same path.

CBSE notes-Light Star light flux Star light flux is the luminous energy emitted per second by the star.

CBSE notes-Light   Condition at which total internal reflection occurs. Total internal reflection occurs if the angle of incidence 'i' exceeds the critical angle 'i c '.

CBSE notes-Light   Why does the sky appear dark instead of blue to an astronaut? The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reach the eyes of the astronauts and the sky appears black to them.

CBSE notes-Light Why does the Sun appear reddish early in the morning? During sunrise, the light rays coming from the Sun have to travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights are scattered out and only longer wavelengths are able to reach our eyes. Since blue colour has a shorter wavelength and red colour has a longer wavelength, the red colour is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.

CBSE notes-Light Explain why the planets do not twinkle? Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified and they do not twinkle.

CBSE notes-Light Why do stars twinkle? Answer: Stars emit their own light and they twinkle due to the atmospheric refraction of light. Stars are very far away from the earth. Hence, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of the atmosphere. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us. Therefore, it appears as if the stars are twinkling at night.

CBSE notes-Light  Far point and near point of the human eye with normal vision? The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm. The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.  

CBSE notes-Light Optical centre The central point in the lens is called the optical centre. If a ray is incident towards the optical centre, it passes undeviated through the lens.

CBSE notes-Light Centre of curvature A lens has two centers of curvature. These are the centers of the two spherical surfaces which combine to form a lens.

CBSE notes-Light Snell’s law Snell's Law relates the indices of refraction of the two media to the directions of propagation in terms of the angles to the normal.

CBSE notes-Light   Distinguish between a real image and a virtual image? The image which can be observed through naked eye is called real image and the image which cannot observed through naked eye is called virtual image.

CBSE notes-Light A beam of red light is allowed to fall in turn on (a) a black cloth (b) red cloth (c) white cloth (d) blue cloth. Describe and explain the appearance in each case. (a) When red light falls on a black cloth it still appears black, because it does not reflect the red colour instead, it absorbs the light falling on it. (b) When red light falls on a red cloth it still appears red, because it reflects only that colour. (c) When red light falls on a white cloth it changes to a red coloured cloth, because the white cloth reflects the red colour of the light. (d) When red light falls on a blue cloth it changes to black coloured cloth, because there is no blue colour in the red light which can be reflected

CBSE notes-Light   Distinguish between a plane mirror, a concave mirror and a convex mirror without touching them? We can distinguish these mirrors by identifying the image of our face produced by them when we just look into those mirrors. (i) A plane mirror will produce an image of the same size as our face. (ii) A concave mirror will produce a magnified image. (iii) A convex mirror will produce a diminished image.

CBSE notes-Light What is the nature, size and position of the image formed of an object at infinity? Explain how it is used as a doctor’s mirror. Solution:When an object is placed at infinity from a concave mirror, the image formed is (i) real and inverted   (ii) highly diminished and   (iii) at the focus When the object is at infinity, the image is formed at the focus of the concave mirror, hence the mirror can concentrate all the parallel rays of the light to its focus. This explains the use of concave mirror as a doctor’s head-mirror. A concave mirror is used as a head-mirror by the doctors to concentrate light on the body parts to be examined.  

CBSE notes-Light What is lateral inversion? Solution:If an object is placed in front of a plane mirror, then the right side of the object appears to be on the left side of the image and the left side of the object appears to be on the right side of its image. This change of sides of an object and its mirror image is called lateral inversion.  

 CBSE notes-Light What does the positive sign associated with the virtual image and the negative sign associated with the real image denote? The positive sign associated with the virtual image indicates the presence of an erect image (above the principal axis of the lens) and the negative sign associated with the real image indicates the presence of an inverted image (below the principal axis of the lens).

CBSE notes-Light Basic laws which determine the reflection of light by mirrors. The reflection of light from mirrors takes place according to the following laws of reflection of light:  (a) the angle of incidence (i) is equal to the angle of refection (r), and (b) the incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.

CBSE notes-Light Name the type of mirror used in the following situations. (a) Headlights of a car (b) Side/rear-view mirror of a vehicle (c) Solar furnace (a) Concave mirror is used in the headlights of a car. This is because concave mirrors can produce powerful parallel beam of light when the light source is placed at their principal focus. (b) Convex mirror is used in side/rear view mirror of a vehicle. Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of it. Because of this, they have a wide field of view. It enables the driver to see most of the traffic behind him/her. (c) Concave mirrors are convergent mirrors. That is why they are used to construct solar furnaces. Concave mirrors converge the light incident on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.

CBSE notes-Light Define 1 dioptre of power of a lens. Power of lens is defined as the reciprocal of its focal length. If P is the power of a lens of focal length F in metres, then      P=1/f The S.I. unit of power of a lens is Dioptre. It is denoted by D. 1 dioptre is defined as the power of a lens of focal length 1 metre. ∴1 D = 1 m−1  

CBSE notes-Light Define the principal focus of a concave mirror. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

MP Board notes-Osmoregulation and excretion Functions of kidney- 1)It excretes harmful and other toxic waste from body 2)It controls blood pressure 3)It maintains the amount of mineral salts in the blood 4)It maintains the amount of water in the body 5)It controls hydrogen ion concentration of the body

MP Board notes-Osmoregulation and excretion                            STRUCTURE OF NEPHRON A.Nephron consists of two parts- Bowman,s Capsule and Secretory tubular part 1)Bowman’s Capsule-The closed end of the nephron comprises a cup shaped structure called bowman’s capsule which contain a bunch of blood capillaries,known as glomerulus.It is situated in the cortex region of the kidney. 2) Secretory tubular part-It is coiled posterior part of Bowman’s c apsule.It has three types- a)Proximal convoluted tubule b)’U’ shape of henle’s loop situated in the medulla c)Distal convoluted tubule opens in collecting tubule of kidney.

MP Board-Osmoregulation and excretion Urinary bladder- A small sac like structure which is situated in a posterior part of abdominal cavity.According to the quantity of urine,it changes shape and position.Its wall consist of non linear muscles.Ureter carries urine or other product from kidney.

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TRANSLOCATION IN PLANTS A. Translocation is necessary as every part of the plant needs food to get energy and for developing his body.The translocation of water,salt and nutrients in plants talks place on the basis of these points- 1)Direction-  a)UPward direction    b)Radial direction    c)Downwardly 2)Path - Xylem and phloem a)Continuity of vascular tissues is distributed by their removal. b)Food materials are send upwardly c)In radial dierection Leaves make food by the process of photosysthesis.The food is in the form of a simple sugar.The food manufactured by the leaves is transported by a tube called phloem and process is called translocation.The movement of material through phloem depends on sieve tubes,which are living cells.These are connected with each other by perforated sieve plates,forming a continuous passage through which food dissolves substance can move freely.Food molecules enter the phloem cells from mesophyll cells of the leaf.After entering they can