How to plot a formula with a given range? - r

Question

I am looking to plot the following:
L<-((2*pi*h*c^2)/l^5)*((1/(exp((h*c)/(l*k*T)-1))))
all variables except l are constant:
T<-6000
h<-6.626070040*10^-34
c<-2.99792458*10^8
k<-1.38064852*10^-23
l has a range of 20*10^-9 to 2000*10^-9.
I have tried l<-seq(20*10^-9,2000*10^-9,by=1*10^-9), however this does not give me the results I expect.
Is there a simple solution for this in R, or do I have to try in another language?
Thank you.

Answer 1

Looking at the spectral radiance equation wikipedia page, it seems that your formula is a bit off. Your formula multiplies an additional pi (not sure if intended) and the -1 is inside the exp instead of outside:
L <- ((2*pi*h*c^2)/l^5)*((1/(exp((h*c)/(l*k*T)-1))))
Below is the corrected formula. Also notice I have converted it into a function with parameter l since this is a variable:
T <- 6000 # Absolute temperature
h <- 6.626070040*10^-34 # Plank's constant
c <- 2.99792458*10^8 # Speed of light in the medium
k <- 1.38064852*10^-23 # Boltzmann constant
L <- function(l){((2*h*c^2)/l^5)*((1/(exp((h*c)/(l*k*T))-1)))}
# Plotting
plot(L, xlim = c(20*10^-9,2000*10^-9),
xlab = "Wavelength (nm)",
ylab = bquote("Spectral Radiance" ~(KW*sr^-1*m^-2*nm^-1)),
main = "Plank's Law",
xaxt = "n", yaxt = "n")
xtick <- seq(20*10^-9, 2000*10^-9,by=220*10^-9)
ytick <- seq(0, 4*10^13,by=5*10^12)
axis(side=1, at=xtick, labels = (1*10^9)*seq(20*10^-9,2000*10^-9,by=220*10^-9))
axis(side=2, at=ytick, labels = (1*10^-12)*seq(0, 4*10^13,by=5*10^12))
The plot above is not bad, but I think we can do better with ggplot2:
h <- 6.626070040*10^-34 # Plank's constant
c <- 2.99792458*10^8 # Speed of light in the medium
k <- 1.38064852*10^-23 # Boltzmann constant
L2 <- function(l, T){((2*h*c^2)/l^5)*((1/(exp((h*c)/(l*k*T))-1)))} # Plank's Law
classical_L <- function(l, T){(2*c*k*T)/l^4} # Rayleigh-Jeans Law
library(ggplot2)
ggplot(data.frame(l = c(20*10^-9,2000*10^-9)), aes(l)) +
geom_rect(aes(xmin=390*10^-9, xmax=700*10^-9, ymin=0, ymax=Inf),
alpha = 0.3, fill = "lightblue") +
stat_function(fun=L2, color = "red", size = 1, args = list(T = 3000)) +
stat_function(fun=L2, color = "green", size = 1, args = list(T = 4000)) +
stat_function(fun=L2, color = "blue", size = 1, args = list(T = 5000)) +
stat_function(fun=L2, color = "purple", size = 1, args = list(T = 6000)) +
stat_function(fun=classical_L, color = "black", size = 1, args = list(T = 5000)) +
theme_bw() +
scale_x_continuous(breaks = seq(20*10^-9, 2000*10^-9,by=220*10^-9),
labels = (1*10^9)*seq(20*10^-9,2000*10^-9,by=220*10^-9),
sec.axis = dup_axis(labels = (1*10^6)*seq(20*10^-9,2000*10^-9,by=220*10^-9),
name = "Wavelength (\U003BCm)")) +
scale_y_continuous(breaks = seq(0, 4*10^13,by=5*10^12),
labels = (1*10^-12)*seq(0, 4*10^13,by=5*10^12),
limits = c(0, 3.5*10^13)) +
labs(title = "Black Body Radiation described by Plank's Law",
x = "Wavelength (nm)",
y = expression("Spectral Radiance" ~(kWsr^-1*m^-2*nm^-1)),
caption = expression(''^'\U02020' ~'Spectral Radiance described by Rayleigh-Jeans Law, which demonstrates the ultraviolet catastrophe.')) +
annotate("text",
x = c(640*10^-9, 640*10^-9, 640*10^-9, 640*10^-9,
150*10^-9, (((700-390)/2)+390)*10^-9, 1340*10^-9),
y = c(2*10^12, 5*10^12, 14*10^12, 31*10^12,
35*10^12, 35*10^12, 35*10^12),
label = c("3000 K", "4000 K", "5000 K", "6000 K",
"UV", "VISIBLE", "INFRARED"),
color = c(rep("black", 4), "purple", "blue", "red"),
alpha = c(rep(1, 4), rep(0.6, 3)),
size = 4.5) +
annotate("text", x = 1350*10^-9, y = 23*10^12,
label = deparse(bquote("Classical theory (5000 K)"^"\U02020")),
color = "black", parse = TRUE)
Notes:
I created L2 by also making absolute temperature T a variable
For each T, I plot the function L2 using different colors for representation. I've also added a classical_L function to demonstrate classical theory of spectral radiance
geom_rect creates the light blue shaded area for "VISIBLE" light wavelength range
scale_x_continuous sets the breaks of the x axis, while labels sets the axis tick labels. Notice I have multiplied the seq by (1*10^9) to convert the units to nanometer (nm). A second x-axis is added to display the micrometer scale
Analogously, scale_y_continuous sets the breaks and tick labels for y axis. Here I multiplied by (1*10^-12) or (1*10^(-3-9)) to convert from watts (W) to kilowatts (kW), and from inverse meter (m^-1) to inverse nanometer (nm^-1)
bquote displays superscripts correctly in the y axis label
annotate sets the coordinates and text for curve labels. I've also added the labels for "UV", "VISIBLE" and "INFRARED" light wavelengths
ggplot2
Plot from wikipedia:
Image source: https://upload.wikimedia.org/wikipedia/commons/thumb/1/19/Black_body.svg/600px-Black_body.svg.png