Science Updates

Flower is the reproductive organ of a plant. A flower may be a monoecious or dioecious, depending on whether the flower has either male or female organ or both the organs respectively. The female organ is known as the Gynaecium and the male organ is known as the Androecium. The female organ consists of the carpel, style and the stigma. The male organ includes stamens and anthers. The carpel is oval shaped and  contains the ovary,which further consists of the female gametes. The style is a stalk-like structure, which helps in transporting the pollen grains landed on the stigma to reach the ovules. The stamen is again a feathery stalk-like structure which holds the anther or box-like structure. Anthers contains the pollem grains or the male gametes,inside the pollen sacs. Most flowers are bright and showy and give nectar and sweet smell to attract insects for pollination. In return of the honey,the insects spread the pollen grains. The transfer of pollen grains from the anther of a flower to the stigma of another flower is known as pollination. Pollination can be of two types:Self and Cross. Self pollination occurs either between the anther and stigma of the same flower or of a different flower on the same plant. Cross Pollination occurs between the stigma of one flower and anther of another flower on 2 different plants. Pollination can occur in insects, wind, water, and animals and can take place both by naturally and by artificial means. Natural Pollination can occur only between the plants which are genetically similar to each other. The fusion of male gametes and female gametes is known as sexual reproduction.

Introduction:

Anything that occupies space and has mass is called matter. Matter can be divided into two types.

1. Pure substance: It consists of single types of particles which are identical and same characteristics.
2. Mixtures: Mixture contains two or more different kind of particles or atoms. The particles show their individual characteristics.

Mixtures are of two types.

• Homogeneous mixtures: A mixture is said to be Homogeneous if all the particles in the mixture are mixed properly, and they cannot be separated by the simple physical process.
• Heterogeneous mixtures: A mixture is said to be Heterogeneous if all the components of the mixture are not uniformly mixed and they can be separated easily.

What are solution and solubility?

A solution is a homogeneous mixture of two or more substances.

The amount of solute present in a saturated solution at a given temperature is known as solubility.

On which factors does the solubility depend on?

• Effect of temperature on solubility: For solid solute in liquid if the temperature increases solubility also increases. For gas solute in liquid if the temperature increases solubility decreases.
• Effect of pressure on solubility: Pressure has no effect on solid solute in liquid. But for the gas solute in liquid solubility increases if pressure increases. This principle is applied in the packaging of cold drinks.

1. Question:

Define reflection of light.

Answer:

The phenomenon by which a ray of light changes the direction of propagation when it strikes a boundary between different media through which it cannot pass is described as the reflection of light.

2. Question:  

Name the two types of reflection.

Answer:   

The two types of reflection are regular reflection and irregular or diffused reflection.

3. Question:

What is meant by diffused reflection?

Answer:   

In this type of reflection the different portions of the surface reflect the incident light in different directions and no definite image is formed but the surface becomes visible.

4. Question:  

Name the phenomenon due to which we see the non-luminous objects.

Answer:   

It is due to diffused or irregular reflection of light that we see the non-luminous objects.

5. Question:  

While looking at your image in a large plane mirror, if you touch your left ear it appears as if your image has touched the right ear. Why?

Answer:   

It is due to lateral inversion that we feel that the image has touched the right ear.

6. Question:  

Name any one device based upon the principle of reflection of light by plane mirrors.

Answer:   

Periscope is a device based on the principle of reflection of light.

7. Question:  

Define magnification.

Answer:   

The ratio of the height of the image to the height of the object is called the linear magnification.

8. Question:  

What type of surface causes regular reflection?

Answer:   

Smooth polished surfaces cause regular reflection.

9. Question:  

Define a) Centre of curvature b) Pole of a spherical mirror

Answer:   

Centre of curvature is the centre of the sphere of which the spherical mirror forms a part. Pole of a mirror is the midpoint of the aperture of the spherical mirror.

10. Question:  

What kind of mirror is used in a car to view the traffic behind it?

Answer:   

Convex mirror is used in a car to view the traffic behind it.

Reflection of Light Rays on a Reflecting Surface:

When a light ray strikes a surface separating two media with different optical properties, part of the light energy is reflected back to the media where it coming from. When light strikes a perfectly reflecting surface, such a mirror for example, all the light energy is reflected.
We first look at the vocabulary used to describe the reflection of light.

The surface separating the two media is called the interface or boundary.
The point at which the incident ray strikes the interface is called the point of incidence.
i is the angle made by the incident ray and the normal to the interface and is called the angle of incidence.
r is the angle made by the reflected ray and the normal to the interface and is called the angle of reflection.

Laws of reflection:

(1) The incident light ray, the reflected light ray and the normal to the interface at the point of incidence make a plane called the plane of incidence

(2) The angle of incidence and the angle of reflection have the same size.

Example 1: A light ray strikes a reflective plane surface at an angle of 56° with the surface.

a) Find the angle of incidence.

b) Find the angle of reflection.

c) Find the angle made by the reflected ray and the surface.

d) Find the angle made by the incident and reflected rays.

Example 2: A ray of light is reflected by two parallel mirrors (1) and (2) at points A and B. The ray makes an angle of 25° with the axis of the two mirrors.

a) What is the angle of reflection at the point of incidence A?

b) What is the angle of reflection at the point of incidence B?

c) If the distance between the two mirrors id d = 4 cm and the length L of the two mirror system is 3 meters, approximately how many reflections take place between the two mirrors?

d) In a real system, at each reflection, there are losses of the light energy travelling between the two mirrors. If L and d are fixed, what can be done to decrease the number of reflections on the mirrors and hence the energy lost by reflection?

Solution:

a) i = 90 - 25 = 75 °

b) the two mirrors are parallel and AB intersect both of them, hence angle r and i' are alternate interior and therefore equal in size. Also r = i (law of reflection). Hence i' = r = i = 75 °

c) If the distance between the two mirrors id d = 4 cm, AC can be calculated as follows
tan(75°) = AC / d = AC / 4
AC = 4 tan(75°) = 15 cm
The number of reflections N that will take place between the two mirrors may be approximated as follows.
N = L / AC = 3 m / 15 cm = 20

d) Let the angle made by the light ray and the axis of the two mirror system be ? instead of 25 °.
The angle of incidence i = 90 - ? = r = i'
tan(i') = tan( 90 - ?) = cot (?) = AC / d
AC = d cot(?)
number of reflections = N = L / AC = L / d cot(?)
If L and d are fixed, the only way to decrease N is to increase cot(?) and hence decrease ?.

Which of the following potential energy curves in figure cannot possibly describe the elastic collision of two billiard balls?

Here r is the distance between centres of the balls:

Answer:

The wave pattern on a stretched string is shown in figure. Interpret what kind of wave this is and find its wavelength?