Lower Secondary Science

[Vivian22]

Hi, I need help with this question:


Given that the refractive index for glass is 1.5 and the refractive index of water is 1, what is the critical angle when a light ray travels from glass to air?

Thanks in advance

[Dr.033430Daniel]

Vivian22:

Hi, I need help with this question:


Given that the refractive index for glass is 1.5 and the refractive index of water is 1, what is the critical angle when a light ray travels from glass to air?

Thanks in advance

Hi Vivian – I always suggest that students start with the general form of Snell’s Law which is

ni (sin i) = nr (sin r)

where ni is the index of refration on the incident side and sin i is the sin of the incident angle with respect to the normal, nr is the index on the refracted side and sin r is the sin of the refracted angle. The refraction is into air with index 1. The refracted angle is 90 degrees (this is the definition of critical angle). The sin of 90 degrees is 1, so the right side of the equation just becomes 1.

So the equation becomes

ni (sin i) = 1

We are given that ni = 1.5 so

sin i = 1/1.5

i = 42 degrees

The critical angle is the incident angle, so this is our answer.

For more questions using Snell’s Law, I have a few posted here....

http://physicsandcalculus.com/2012/snells-law-problems

[Vivian22]

Thank you very much Dr. Daniel :goodpost:

[Vivian22]

Hi, I need help with a few Chemistry questions


1) Which of the following shows the number f atoms present in 6 dm^3 of ethene at room temperature and 1 atmosphere?
A) 1.5 x 10^23
B) 3 x 10^23
C) 6 x 10^23
D) 9 x 10^23

2) In 10g of the following halogens, which has the most number of particles?
A) chlorine
B) bromine
C) fluorine
D) iodine

3) An organic compound P reacts with sodium oxide to give a compound with formula C3H5O2Na. WHich of the following is compound P?
A) ethanol
B) propanol
C) ethanoic acid
D) propanoic acid.

Thanks

[Dr.033430Daniel]

Vivian22:

Hi, I need help with a few Chemistry questions


2) In 10g of the following halogens, which has the most number of particles?
A) chlorine
B) bromine
C) fluorine
D) iodine

Thanks

C) Flourine. If you look at the periodic table of the elements you can see the atomic mass of each element. Flourine has the lowest atomic mass for any element on this list, so 10g will contain more particles.

[Dr.033430Daniel]

[quote="Vivian22"]Hi, I need help with a few Chemistry questions


1) Which of the following shows the number f atoms present in 6 dm^3 of ethene at room temperature and 1 atmosphere?
A) 1.5 x 10^23
B) 3 x 10^23
C) 6 x 10^23
D) 9 x 10^23


A.\tEthene has a molecular weight of 28 grams per mole. It has a density of 1.18 kg/m3.
6dm3 = 0.006m3. Multiply that by the density and you get 7.08 grams. Divide this by 28 and you have 0.25 moles. A mole is 6.02 X 10 ^23 atoms. 0.25 moles is 1.5 x 10^23 answer A.

[Chan09]

Are there any recommended sec1 or 2 science guide books that would be useful as I find it hard to know which ones to use as the sec schools vary as I am doing o levels in 2016? The ones I see are from MC or GLM though not sure if school notes are sufficient for lower sec? Tks

[Vivian22]

Thanks again Dr Daniel

[milkbun]

Chan09:

Are there any recommended sec1 or 2 science guide books that would be useful as I find it hard to know which ones to use as the sec schools vary as I am doing o levels in 2016? The ones I see are from MC or GLM though not sure if school notes are sufficient for lower sec? Tks

Do share with me if there's any good ones cos my boy going sec 1 next year.

Anyway, his tutor provides very good notes and materials...so dont really need to worry...but just being kaisu here...tt's why i'm a kaisu parent.

:evil:

[LarryG67]

For science, my son uses these book from popular by the ‘Education Publishing House’ publisher. Quite concise and clear.